1/2 cup all-purpose flour
1/4 cup unsweetened cocoa powder
4 teaspoons instant espresso
1 teaspoon baking powder
1/8 teaspoon salt
4 tablespoons unsalted butter
2/3 cup packed light-brown sugar
1 large egg
4 ounces bittersweet or semisweet chocolate, melted and cooled
1 tablespoon milk
Confectioners' sugar, for coating
In a medium bowl, sift together flour, cocoa, espresso, baking powder, and salt. With an electric mixer, cream butter and brown sugar until light and fluffy. Beat in egg until well combined; mix in cooled chocolate. With mixer on low, gradually add flour mixture; beat in milk until just combined. Flatten dough into a disk; wrap in plastic. Freeze until firm, about 45 minutes.
Preheat oven to 350 degrees. Line two baking sheets with parchment. Shape dough into 1-inch balls. Pour confectioners' sugar (about 1/2 cup) into a medium bowl; working in batches, roll balls in sugar two times, letting them sit in sugar between coatings.
Place on prepared baking sheets, 2 inches apart. Bake until cookies have spread and coating is cracked, 12 to 14 minutes; cookies will still be soft to the touch. Cool cookies on a wire rack.