In a medium bowl, stir together the flour, cocoa, espresso, baking powder, and salt.

In the bowl of your mixer, cream together the butter and sugar until light and fluffy. Beat in the egg until combined and mix in the cooled chocolate. Gradually add in the flour mix. Beat in the milk until just combined.

Flatten dough into a disk and wrap in plastic wrap. Freeze for 45 minutes.

Preheat oven to 350 degrees. Line a baking sheet with parchment paper.

Shape dough into 1-inch balls and roll in the powdered sugar. After you've rolled all of the balls in the sugar, roll them in again one by one. Each ball of dough should get two coatings of sugar.

Place on cookie sheet 2 inches apart. Bake until the cookies have spread and the coating is cracked, 12-14 minutes. Cool on a wire rack.