1.In a medium bowl, sift together the flour, baking powder, and salt. Set aside.
2.In the bowl of a stand mixer with paddle attachment, cream together butter, powdered sugar, and sugar on medium speed for at least 2 minutes, scraping down the sides, as needed.
3.Add the egg, vanilla, and milk and blend until smooth and fluffy, about 30 seconds. Add the flour mixture and mix on low until fully incorporated. Cover and chill the batter for 10-15 minutes.
4.Preheat oven to 350°F. Scoop 1/2 teaspoon-sized dollops of the batter, about 1 inch apart onto a baking sheet lined with parchment. Bake 12-15 minutes, rotating pan halfway through baking. Cookies will be golden brown.
5.Transfer cookies to a wire rack and allow to cool completely before serving.