Preheat oven to 350 degrees F. Line two heavy, light or medium colored baking sheets with parchment paper.: You should notice a faint warmth in the air as the oven comes up to temperature, and this preheating ensures consistent oven spring and even baking. The reason it matters is that starting with the correct temperature gives the cookies the lift they need before the edges set, so they bake with pleasant rounded tops rather than spreading into thin discs. A common mistake is skipping full preheat, which can produce unevenly baked cookies. If your oven runs hot, rotate the baking sheet halfway through to avoid overly browned edges.
In a bowl, whisk together flour, baking soda, cream of tartar, and salt and set aside.: When you line pans, you'll notice a smoother surface for cookies to glide on and easier cleanup after baking. Using parchment prevents sticking and encourages even browning on the bottoms. I prefer medium colored sheets to avoid over browning when using thin pans. A frequent slip is placing cookies on warm sheets, which can start spreading early; always cool pans between batches if needed.
In a mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, cream butter, shortening, and sugar until light and fluffy, 2 to 3 minutes. Add egg and egg yolk and beat on high speed for 30-60 seconds until smooth. Mix in vanilla.: As you whisk, pay attention to the flour becoming evenly speckled with the leaveners and salt, which signals uniform distribution. This step ensures every bite rises and seasons consistently. Poorly mixed dry ingredients can create pockets of unbalanced flavor or inconsistent texture, so whisk until homogeneous.
Dump in dry ingredients and mix on low speed until incorporated and no streaks of dry ingredients remain. Add 2/3 cup M&Ms and fold until evenly distributed.: You will see the mixture lighten in color and increase slightly in volume, taking on a pale, whipped look, and the sugar will become well incorporated. This creaming traps tiny air bubbles that help with the cookies' tender lift. Overbeating can produce too much air and cause collapse, while undercreaming leaves a dense texture, so aim for 2 to 3 minutes.
Scoop dough into 50 g balls (a generous scoop of dough with a medium #40 cookie scoop, or approximately 2 tablespoons worth). Roll into a ball between your palms, then roll in a small bowl filled with granulated sugar until evenly coated.: The batter will become glossy and cohesive, and you'll hear a steadier, smoother mixing sound indicating full incorporation. Eggs add structure and moisture, making the dough pliable and rich. Mixing too long can overwork the batter, so stop once it looks smooth and uniform.
Place on cookie sheet with 2 to 3 inches of space between cookies to allow for spreading. Press a few more M&Ms into the top of each dough ball for a prettier baked cookie (you can place them pretty close together as they will spread out in the oven).: The aroma of vanilla will lift the mixture, adding warmth and depth. Stirring this in helps distribute flavor evenly, and it contributes to the cookie's inviting scent while baking. Forgetting to add vanilla will mute the overall flavor profile, so be deliberate about this small but impactful addition.
While you can bake the cookies immediately without chilling, I recommend popping the entire cookie sheet in the fridge for 10 to 15 minutes; this will help the cookies bake more evenly and not spread quite as much. If you find your cookies are spreading too much still, chill the dough balls for up to 30 minutes as needed.: At this stage, you'll see the dough come together and thicken, and the dusty flour will disappear into the batter. Mixing gently protects the cookie's tender crumb by avoiding gluten overdevelopment. A common error is mixing too vigorously, which can make the cookies tough, so keep the mixer on low and stop as soon as the streaks are gone.
Bake for 12 to 13 minutes (10 to 11 minutes if baking from room temperature dough) or until tops are puffed and edges are just barely golden brown. I recommend checking on the cookies at around 9 minutes; rotate the cookie sheet, also give it a good hard rap on the oven rack once or twice to deflate the tops and ruffle the edges, then bake for a few more minutes as needed.: Folding should be gentle and purposeful, so you feel pockets of colorful candy distributed without breaking them. The visual cue is an even scattering of little chocolate dots throughout the dough. Overfolding crushes the candies and bleeds color into the dough, while underfolding leaves some cookies without M&Ms, so fold until even.
Immediately after removing from the oven, swirl a large round cookie cutter around the warm cookies, tucking in the uneven edges to create perfectly circular cookies. Let cool on cookie sheets for 15 minutes, then transfer to a wire rack to cool completely.: As you portion, notice the dough's tacky, slightly soft texture that holds shape when scooped. Rolling into smooth balls helps them bake evenly, and using a consistent 50 g size produces uniform cookies. A common mistake is inconsistent scoops, resulting in uneven bake times, so weigh or use a consistently sized scoop for uniformity.
Freshly baked cookies are best enjoyed the same day, or refrigerated in a bag or airtight container for up to 3 days. For longer storage, wrap tightly in plastic wrap and place in a zip top bag in the freezer for up to a month. Let come to room temperature for 30 minutes before serving.: The sugar coating gives a sparkling crust and a delicate crunch. You will feel the sugar crystals adhere and see the surface take on a slight sheen. Rolling too roughly can flatten the dough, so be gentle to preserve the shape.
Place on cookie sheet with 2 to 3 inches of space between cookies to allow for spreading: Arrange the dough balls so the cookies have room to expand and bake into neat rounds. Visually, you want equal spacing and no touching. Overcrowding leads to merged cookies, so leave adequate distance and use multiple sheets if necessary.
Press a few more M&Ms into the top of each dough ball for a prettier baked cookie: Pressing extras creates an attractive mosaic on top after baking and adds extra chocolate on the surface. Do this gently so the candies remain visible without being pushed too deep. Be careful not to press so hard that the cookie shape is distorted.
Chill the entire cookie sheet in the fridge for 10 to 15 minutes: The chill firms the fats and helps control spreading, resulting in thicker cookies with rounded tops. You will notice the dough feel cooler and slightly firmer to the touch after chilling. A common oversight is skipping chill entirely; unchilled dough often spreads more, yielding thinner cookies.
Bake for 12 to 13 minutes: Watch for puffed tops and just barely golden edges, and expect the aroma of warm butter and sugar to fill the kitchen. The middle should look set yet soft to the touch, which signals doneness. Overbaking will dry them out, so remove them when edges are light gold and centers are still slightly soft.
Check at around 9 minutes; rotate the cookie sheet, rap the oven rack once or twice to deflate tops and ruffle edges: This trick encourages even browning and gives the cookies a classic ruffled edge. You will hear a slight clatter when you rap the rack, and you should return the tray to finish baking. Missing this step can leave domed tops that crack unevenly.
Immediately after removing from the oven, swirl a large round cookie cutter around the warm cookies: The cutter smooths uneven edges into perfect circles while the cookies are malleable, and you can see the warm chocolate glisten as the surface relaxes. Pressing too hard can flatten them, so use a gentle swirling motion to tuck edges back in.
Let cool on cookie sheets for 15 minutes, then transfer to a wire rack to cool completely: During this resting period, the cookies finish setting and firm up enough to move without breaking. You will smell the lingering sweetness and notice the chocolate begin to set. Moving them too soon can cause tearing, so wait the full 15 minutes.
Store freshly baked cookies in a bag or airtight container for up to 3 days: Stored properly, the cookies maintain a pleasant texture for several days, and refrigeration can slow staling. For longer storage, wrap tightly and freeze for up to a month, then bring to room temperature for 30 minutes before serving. A common mistake is leaving them unsealed, which leads to rapid softening or drying.
