Add the butter to the bowl of a stand mixer fitted with the paddle attachment. With the mixer on low speed, slowly add the powdered sugar.
1 cup unsalted butter, 1 cup powdered sugar
In a small bowl, mix the flour and cocoa powder together. With the mixer still on low speed, slowly add the dry ingredients to the butter mixture,
1¾ cups all-purpose flour, ¼ cup unsweetened cocoa powder
Scrape down the sides of the bowl and mix just until no large lumps of flour remain. The dough should come together in large crumbles– if it is too dry, add 1 tablespoon of milk and mix to combine. If the dough still doesn’t come together, add the second tablespoon of milk.
2 tablespoons milk
Shape the dough into a disc and wrap it tightly with plastic wrap. Place it in the refrigerator to chill for 15 minutes.
When the dough is chilled, place it on a lightly floured surface and roll it to ¼-inch thick. Cut out as many 1½-inch round cookies as you can.
Use your thumb or a spoon to create a small well in the center of each cookie.
Place the cookies on a parchment-lined baking sheet and put them into the freezer to chill for 15 minutes.
While the cookies chill, preheat oven to 350°F.
Bake the chilled cookies for 12-15 minutes, or until the edges are set and the centers are no longer soft. If the wells in the center puff too much while baking, use a spoon to reshape them while the cookies are still warm.
Allow the cookies to cool completely before adding the ganache.