Preheat the oven to 325 degrees.
Add the almond flour, coconut flour, baking powder, confectioners sugar substitute, salt, butter, eggs, and vanilla to a mixing bowl.
Beat on high until well combined.
Press the dough into an even layer in a well-greased 9x9 baking dish.
Bake for 25 minutes, then remove from the oven and cool to room temperature.
Prepare a batch of the vanilla buttercream icing.
Layer the icing on top of the cooled cookie base.
Add some sugar free sprinkles to the top, then chill in the refrigerator for at least an hour before slicing.