Preheat oven to 375 F. Grease or use parchment paper to line a 9x13-inch metal baking pan. If using a glass pan, preheat the oven to 350F. Set aside.
To make the sugar cookie bars, beat butter, granulated sugar, and brown sugar together until fluffy and well-combined.
Add egg and egg yolks. Beat until eggs are well-incorporated, about 2-3 minutes.
Stir in vanilla and almond extract, if using.
Add flour, baking powder, and salt. Stir until just incorporated.
Add batter to prepared pan.
Bake for about 15-20 minutes, or until the edges are very light golden brown. A toothpick inserted into the center should come out clean. It's better to slightly underbake the bars than to overbake them.
Let the bars cool in the pan until they reach room temperature.
To prepare frosting, beat butter until fluffy.
Add powdered sugar and salt, and mix until combined.
Stir in vanilla extract.
Mix in enough cream to make a spreadable frosting.
Butter - I use salted butter. If using unsalted butter, increase the salt to ¾ teaspoon in the bars and add an extra pinch or two to taste to the frosting.
Brown sugar - I recommend using light brown sugar in this recipe. The light brown sugar helps to add moisture and chew to the bars without giving them any molasses flavor.
Egg yolks - The fat in the egg yolks helps to give the bars a chewier texture.
Vanilla extract - Sugar cookies are basically butter, flour, sugar, and a few other ingredients. Feel free to increase the amount of vanilla extract in the bars, frosting, or both to bump up the vanilla flavor.
Almond extract - I like to add a bit of almond extract to give the bars just a bit more flavor. It's an optional ingredient and can be omitted if you prefer.
All-purpose flour - It's important to properly measure the flour. Too much flour can give you dry, crumbly bars. To properly measure the flour, either weigh it or sift or stir it to break it up. Then, lightly spoon the flour into the measuring cup and level.
Powdered sugar - If your powdered sugar if lumpy, sift or stir it before adding it to help break it up. This will help it better incorporate with the butter.
Heavy cream - I like to use heavy cream because it makes for a richer, creamier frosting. You can use milk if you prefer.