Preheat the oven to 350 degrees. Line a baking sheet with parchment paper, or a silicone liner.
In a medium bowl, whisk together the dry ingredients, the flour, cocoa powder, baking powder, and salt. Set the mixture aside. 1 1/2 cups all purpose flour, 1/2 cup unsweetened cocoa powder, 1/2 tsp baking soda, 1/2 tsp baking powder, 1/2 tsp salt
Using a large bowl with a hand mixer, or a stand mixer, cream together the butter and sugars for a couple minutes until light and fluffy. 1/2 cup unsalted butter, 1/2 cup granulated sugar, 1/2 cup brown sugar
Add in the egg, and vanilla extra and stir until just combined. Scrape the bottom and sides of the bowl as needed. 1 large egg, 1 tsp vanilla extract
Add the dry ingredients to the wet ingredients and mix on slow speed until combined. Add in the milk if needed, if the dough is too dry. 1 TBS milk
Scoop the dough into 1 TBS portions and roll them into balls. Roll the balls into the remaining sugar. 1/4 cup granulated sugar
Place them onto the prepared baking sheet, 2 inches apart from each other. Press an indent into the top of each cookie with the back of a teaspoon measuring spoon.
Bake the cookies for 5 to 7 minutes, until the tops are just barely set. Remove them from the oven, and re-impress the indent into the top of each cookie.
Let the cookies sit on the cookie sheet for 10 minutes, then transfer the cookies to a wire rack to cool more.