line a sheet pan with parchment or spray with cooking spray and set aside.
cream the butter and sugars in the bowl of a stand mixer fitted with a paddle attachment on medium-high speed for 2-3 minutes until fluffy and pale yellow. stop the mixer and scrape down the sides of the bowl with a spatula.
with the mixer on low speed, add the egg yolk and then increase the speed to medium and beat for 1-2 minutes until the mixture is pale white in color.
continuing to keep the mixer on low speed, add the remaining ingredients and mix for 1-2 minutes until all dry ingredients have been incorporated. stop the mixer and scrape down the sides of the bowl with a spatula.
place the cookie dough in the center of the prepared pan and spread out until it is 1/4 inch thick (it will not cover the entire pan).
bake in the preheated oven for 15 minutes or until puffed slightly but set firmly. cool while preparing the filling.