In a medium bowl, stir together the flour, ¾ cup sugar, cocoa powder, baking powder and salt. Add the espresso mixture, butter, and eggs and stir until well combined. Cover and refrigerate the dough for at least 30 minutes.

Preheat the oven to 350˚F.  Line 2 baking sheets with parchment paper.

Roll 2 tablespoons of dough into balls. Roll each ball in the 1/3 cup of sugar, then in the ½ cup powdered sugar.  Place cookies 2 inches apart on baking sheets. Lightly flatten each cookie with your hand and make a deep, inch-wide indentation in the center of each cookie with your thumb.

Bake cookies until puffed and cracked, about 8 to 10 minutes. Allow to cool 5 minutes on baking sheets. Transfer to wire racks and allow to cool completely.

To make filling, combine the sugar and milk in a medium bowl.  Place filling in a plastic sandwich bag. Snip off a tiny piece of one of the corners. Pipe filling into the indentation of each cookie. Decorate with crushed mint candy.