Design Eat Repeat

0 Preheat oven to 350℉. 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 the unsalted butter (1 cup, i.e. 2 sticks) and granulated sugar (1 cup) on medium speed for 20-30 seconds. While the mixer is still on medium speed, add in egg (1 large), vanilla extract (2 teaspoons), and lemon extract (1 tablespoon).
2 Once mixed, add in baking powder (1 ½ teaspoons), salt (½ teaspoon), and all-purpose flour ½ cup at a time on low speed (2 ¼ cups total). Mix until all flour disappears, typically 20-30 seconds. You do not need to beat/mix this any longer than it takes to mix in the flour.*Be sure to measure your flour properly using the spoon & level method, as adding too much can result in dry bars.  
3 Spray a metal 9×13 pan with cooking spray, then line pan with parchment paper (having the grease underneath the paper will help the paper stay down).
4 Press dough into an even layer in pan.
5 Bake bars at 350℉ for 14-16 minutes. The trick to soft sugar cookie bars is to take them out right when you see the TINIEST amount of brown around the edges. The center may look underdone and this is okay; the bars 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 (½ cup, i.e. 1 stick) 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 (1 teaspoon), lemon extract (1 ½ teaspoons), and powdered sugar (2 cups). Once incorporated, turn the mixer to high speed and beat for 2 minutes until light and fluffy. Add fresh lemon juice (1 tablespoon) and beat for 30 seconds. If desired, add a drop of yellow food coloring.* 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. Cut into 24 squares.
15 Place in airtight container and store in for 2-3 days (they will keep longer, but will start to dry out after about 3 days). These bars are also amazing frozen and will keep in the freezer for 1-2 months (any longer and they tend to dry out).