Preheat oven to 350 degrees F.
In a medium bowl, sift together the dry ingredients: all-purpose flour, baking powder, and salt. Set aside.
In a large bowl or in the bowl of a stand mixer, cream together butter, powdered sugar, and granulated sugar on medium speed, until light and fluffy. This may take 2-3 minutes, make sure to scrape down the sides and bottom of the bowl with a rubber spatula.
Add the egg, vanilla, and milk and beat until smooth and fluffy, about 30 seconds to 1 minutes.
Next, add the flour mixture to the butter mixture and beat just until combined.
Cover the bowl with parchment paper and chill it in the fridge for 10-15 minutes.
Line a cookie sheet with parchment paper.
Scoop 1/2 teaspoon-sized dollops of batter 1 inch apart onto the cookie sheet.
Bake for 12-15 minutes, rotating the baking sheet halfway, the cookies should be lightly golden brown.
Cool completely before serving.