Preheat the oven to 350 degrees Fahrenheit and line 2-3 baking sheets with parchment paper.: As the oven warms you will notice a faint warmth spreading through the kitchen, which sets the stage for even baking. This temperature encourages the cookies to set their edges quickly without browning too much. Make sure your oven racks are centered, and avoid opening the door repeatedly, because that can cause uneven baking. A common mistake is not lining the pans, which leads to sticking and uneven bottoms.
In a medium bowl, whisk together the 2 cups all-purpose flour, 1/2 teaspoon salt, and 1/2 teaspoon baking powder. Set aside.: The dry ingredients will look uniform and pale, and whisking them aerates the flour which helps with even mixing later. I always check for lumps in the flour and remove any clumps gently with the whisk. If you skip this, you might end up with pockets of flour that show up in the baked cookie texture.
In another medium bowl, whisk together the 1/2 cup softened unsalted butter and 2/3 cup granulated sugar for 2-3 minutes until light and fluffy. Add the 2 egg yolks, 2 tablespoons sour cream, 1 1/2 tablespoons vanilla extract, and 1 teaspoon lemon extract, and mix until just combined.: You will see the mixture transform to a paler, airy cream as air is incorporated, and the sugar begins to dissolve into the butter. This step sets the base for tenderness, and over mixing can make the butter too soft, which affects shape retention. Stop as soon as the additions are incorporated to keep a tender crumb. A mistake is using cold butter which will not cream properly, leaving a gritty texture.
Add the flour mixture to the butter mixture and mix until just barely combined.: At this stage you should see streaks of flour disappear into a cohesive dough, but avoid overworking it, because too much handling develops gluten and makes the cookies tough. The dough should hold together when pressed but not feel gummy. If it seems crumbly, a teaspoon of cold water can help, but add sparingly to avoid softening the dough too much.
Split the dough into 2-3 pieces and roll them one at a time on a lightly floured surface until they are 1/4 inch thick. Use a Christmas tree cookie cutter (or alternative shapes!) to cut out the cookies.: Rolling one piece at a time keeps the dough cold and easier to manage, and the surface should be smooth with minimal sticking. Look for even thickness so cookies bake uniformly. If the dough warms and gets sticky, pop it in the fridge for a few minutes. A frequent error is rolling too thin which leads to brittle cookies that brown quickly.
Press the scraps together and set them aside to rest for about 5 minutes. Then re-roll the scraps and cut more cookies until all the dough has been used.: Resting relaxes the gluten and prevents the dough from springing back excessively, keeping shapes neat. When you re-roll, try to be gentle to avoid overworking, which can make the cookies tough. If you see cracks, lightly press to seal them before cutting. Avoid re-rolling too many times, as repeated handling warms the butter and changes texture.
Place the cookies on the parchment-lined cookie sheets, leaving 1 1/2 inches between them.: Give them space so hot air flows around each cookie, which helps edges set evenly. You should be able to see a uniform pale dough on the sheets, and arranging them with space prevents them from merging as they expand slightly. A common oversight is crowding the pan, which results in misshapen cookies.
Bake the cookies for 7-8 minutes, rotating the pan in the oven after 4 minutes to ensure they bake evenly. The edges will be just set and the cookies will be very light in color and should have very minimal coloring underneath them.: You will notice a faint aroma of butter and vanilla as they near doneness, and the tops will look matte rather than glossy. Rotate the pan to counteract hot spots in the oven so all cookies finish together. If you wait for browning, the cookies will lose the delicate, pale look that makes them so attractive, so remove them when they are just set.
Let the cookies cool on the pan for 2 minutes for the shape to set before transferring to a cooling rack. Let the cookies cool completely before frosting.: The short rest lets the cookies firm up so they retain their crisp outlines when moved, and transferring too soon risks deforming them. Cool fully so the frosting does not slide off, you should feel them at room temperature before decorating. Rushing to frost warm cookies is a common mistake that causes smeared decorations.
In the bowl of a stand mixer, whisk the 1 (8-ounce) block softened cream cheese and 1/2 cup softened unsalted butter together until smooth. Scrape down the bowl and beat again for about 10 seconds to make sure everything is completely incorporated.: The frosting base should become luxuriously smooth, with a slightly glossy sheen that signals it is well blended. The taste will be tangy and rich, a lovely contrast to the sweet cookie. If you do not fully soften the cream cheese , the frosting can be lumpy, so ensure it is at room temperature before starting.
Add the 1 teaspoon vanilla extract, 1/2 teaspoon salt, and 4 cups powdered sugar. Mix on low until the powdered sugar is moistened enough to increase the speed without it making a mess. Beat until smooth.: Start slow to avoid a powdered sugar cloud, then increase speed once the sugar is incorporated and the frosting becomes thick and pipeable. The mixture should be homogenous and creamy, without graininess. If it is too loose, add a little more powdered sugar, and if too stiff, a splash of milk or cream can help, but add sparingly to avoid softening it too much.
Color the frosting and decorate the cooled cookies.: When you tint the frosting, mix small amounts of gel color to reach vivid hues without thinning the frosting. Decorating is where personality shines, from simple spreads to piped outlines and festive dots. Work with small batches if you want multiple colors, and store finished cookies on a flat surface until the frosting sets. One mistake is using liquid food coloring which can water down the frosting, choose gels for best results.
