Design Eat Repeat

0 Preheat oven to 350°F. If butter hasn't been softened you can place in microwave for 10 seconds to *slightly* soften it.
1 In a large bowl, cream together unsalted butter ([adjustable]1 cup, i.e. 2 sticks) and granulated sugar (1 cup[/adjustable]) on medium speed for 20-30 seconds; do not overbeat. While mixer is still on medium speed, add in egg ([adjustable]1 large), vanilla extract (2 teaspoons), and lemon extract (1 tablespoon[/adjustable]).
2 Once mixed, add in baking powder ([adjustable]1 ½ teaspoons), salt (½ teaspoon[/adjustable]), and all-purpose flour ½ cup at a time ([adjustable]2 ¼ cups total[/adjustable]). Be sure to measure your flour properly using the spoon & level method, as adding too much can result in a dry cookie cake. Mix until all flour disappears, typically 20-30 seconds.
3 Spray a round metal 12" cookie cake pan with cooking spray, then line the bottom with parchment paper (having the grease underneath the paper will help the paper stay down).If you don’t have a cookie cake pan, bake in a 9x13 pan for 14-16 minutes or use this DIY cookie cake pan hack to make one from aluminum foil and bake it for a bit longer.
4 Spread dough into an even layer in pan. If the dough is sticky, you can place another piece of parchment paper on top and continue pressing down. 
5 Bake cookie cake at 350°F for 16-18 minutes until edges are lightly browned. The trick to a soft sugar cookie cake is to take it out right when you see the TINIEST amount of brown around the edges. The center may look underdone and this is okay; the cake will continue to bake in the hot pan when cooling.
6 Allow to cool completely before frosting (you can put the pan in the fridge on a cooling rack to speed this up).
To Make Frosting:
9 In a large bowl, beat unsalted butter ([adjustable]½ cup, i.e. 1 stick[/adjustable]) on medium speed using a hand or stand mixer (with paddle attachment) for 1-2 minutes (until light and creamy).
10 Keeping the mixer on medium speed, slowly add in vanilla extract ([adjustable]1 teaspoon), lemon extract (1 ½ teaspoon), a pinch of salt, and powdered sugar (2 cups[/adjustable]). Once incorporated, turn the mixer to high speed and beat for 2 minutes until light and fluffy. Add the fresh lemon juice ([adjustable]1[/adjustable] tablespoon, optional) and beat for 30 seconds. If desired, add a drop of yellow food coloring.
16 Add an additional tablespoon of juice if the frosting seems too thick to spread. (*If omitting the fresh lemon juice (or if your frosting is too thick to spread), add a splash of milk to help thin the frosting.))
11 If using sprinkles, add them immediately after frosting. If you wait too long, the frosting will set and the sprinkles won't stick.
15 Storage: If not eating same day, cover with plastic wrap and store in refrigerator for 2-3 days. This sugar cookie cake is also amazing frozen for 1-2 months (any longer and it tends to dry out).