Whisk the flour, baking powder, and salt in a small bowl. Set aside.
Combine the butter, confectioner’s sugar, and granulated sugar in a large mixing bowl. Beat until the mixture is smooth and the butter has absorbed the sugars.
Beat in the egg, vanilla, and milk. Mix well.
Slowly stir in the dry mixture, adding a little at a time to work each addition into the wet ingredients before adding more until it has been fully combined.
Cover and chill in the refrigerator for 15 minutes.
Once cold, drop ½ tsp-sized balls of dough onto the prepared baking sheet about 1” apart.
Bake for 12-15 minutes, rotating the baking sheet halfway through. The tops of the cookies will be light yellow while the bottoms will be slightly browned.