In a medium bowl, combine the flour, baking powder, cocoa, and salt. Set aside.
1 cup all-purpose flour, ½ teaspoon baking powder, ½ cup unsweetened cocoa powder, ½ teaspoon kosher salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars together until light and fluffy, about 2 minutes. Mix in the egg and vanilla.
½ cup unsalted butter, ½ cup brown sugar, ½ cup granulated sugar, 1 large egg, 1 teaspoon pure vanilla extract
Add the flour mixture and mix to combine. Beat in the milk. The cookie dough will be quite thick.
1 tablespoon milk
Fold in the white chocolate chips.
1 cup white chocolate chips
Cover the dough with plastic wrap and chill for at least 3 hours.
Remove cookie dough from the fridge and allow to sit at room temperature for 10-15 minutes.
Preheat oven to 350°F. Line a baking sheet with parchment paper.
Shape the dough into 16 balls. Place the dough balls on the baking sheet at least 2 inches apart. With your palm, gently press down on each dough ball to flatten slightly. Top the cookies with additional white chocolate chips.
¼ cup white chocolate chips
Bake for 12-15 minutes. Transfer the cookies to a wire rack to cool completely.