Preheat oven to 350°F. Line a 9" round cake pan with foil and spray with nonstick cooking spray.: As the oven warms to 350°F you will notice a dry, warm scent filling the kitchen, which signals the right environment for even baking. Lining the pan with foil and spraying with a nonstick agent ensures the cake releases cleanly, preventing tearing when you lift it out. I press the foil smoothly into the pan so there are no air pockets that could create uneven browning. Common mistakes at this stage include not preheating long enough, which causes the cookie cake to bake unevenly, or skipping the spray, which can lead to sticking and a ruined presentation.
Make the cookie cake: Beat butter and sugar with a hand or a stand mixer until creamy. Beat in egg and vanilla, then mix in baking soda, salt, and cream of tartar. Slowly mix in flour. Stir in M&Ms and sprinkles.: Beat butter and sugar with a hand or a stand mixer until creamy. Beat in egg and vanilla, then mix in baking soda, salt, and cream of tartar. Slowly mix in flour. Stir in M&Ms and sprinkles. : When creaming the softened unsalted butter and granulated sugar , listen for a change in sound from a heavy thud to a lighter, more airy whirr, and watch for the color to turn pale, which indicates air has been incorporated. Adding the egg and vanilla extract should produce a glossy, cohesive batter. Mixing in the baking soda, salt , and cream of tartar distributes the leavening so the crumb rises evenly; fold in the all purpose flour slowly to avoid developing too much gluten. Finally, fold in the M&Ms and sprinkles gently so the candy shells do not shatter and color does not bleed excessively. A frequent error is overmixing after adding flour, which leads to a dense texture rather than the tender chew you want.
Press batter into prepared pan, pressing more M&Ms onto the top as desired. Bake for 19-22 minutes or until the edges are just turning a light golden brown. It’s better to under bake this dough than over bake it. The cake will continue cooking a bit as it cools so err on the side of underdone for the best tasting cookie. Let it cool completely before frosting.: As you press the batter into the pan you should feel it become evenly compacted and see a smooth, even surface with candy peeking through. Pressing a few extra M&Ms on top creates attractive finished pockets of color. In the oven, watch the edges for a light golden brown and a faint aroma of baked butter and sugar; these cues tell you it is done. The center will still be slightly soft, which is okay because residual heat will finish the set. Overbaking will dry the cake and dull the chew, so remove it when slightly under to maintain moistness. Cooling completely prevents the frosting from melting and sliding off, so resist the urge to frost while warm.
Make the frosting: beat butter until smooth, then slowly beat in powdered sugar until crumbly. Add salt and vanilla, then add 1 tablespoon of heavy whipping cream. Beat until smooth, adding more a teaspoon at a time to get a spreadable texture.: beat butter until smooth, then slowly beat in powdered sugar until crumbly. Add salt and vanilla, then add 1 tablespoon of heavy whipping cream. Beat until smooth, adding more a teaspoon at a time to get a spreadable texture. : When creating the frosting, the unsalted butter should be pale and silky, and the powdered sugar should blend into a fluffy, snow like mass. Adding the vanilla extract infuses the frosting with warm aroma, and the pinch of salt balances intense sweetness. Begin with 1 tablespoon of heavy whipping cream and add by teaspoon until the frosting reaches a spreadable but stable consistency. If the frosting is grainy it likely needs more beating or sifting of the powdered sugar . Avoid adding too much liquid at once, which will make the frosting runny and difficult to pipe.
To frost: use a 1 M tip and a ziplock or piping bag to decorate the cookie cake. Top with more sprinkles and M&Ms.: use a 1 M tip and a ziplock or piping bag to decorate the cookie cake. Top with more sprinkles and M&Ms. : When piping with a 1 M tip the piping bag should feel comfortable in your hand, and pressure should be steady to create uniform shells or rosettes. The frosting should hold shape against the warm cookie surface, and additional M&Ms and sprinkles can be added for texture and color contrast. If your frosting slides, it is likely too soft or the cake is not fully cooled. A quick fix is chilling the cake briefly before icing or chilling the frosting slightly to firm it up. Keep the piping motions fluid, and vary placement for a charming, handmade look.
Store loosely covered for up to 2 days.: After decorating the cake, store it loosely covered at room temperature to preserve the frosting texture and the cookie crumb. The cake will stay tender for about 48 hours, and you will notice the M&Ms soften slightly, which some prefer for easier slicing. Avoid airtight containers that can trap moisture and make the sprinkles lose their crunch. If you need to keep it longer, refrigeration is possible but may slightly change the frosting texture and cake tenderness.
