Preheat oven to 350° degrees Line 2 baking sheets with parchment paper or a silicone baking mat. Set aside.

In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, salt, and espresso powder. Using your fingers, rub butter into flour mixture until coarse, sand-like crumbs are formed. Stir in chopped chocolate.

In a small bowl, whisk together egg, vanilla and peppermint extract. Using a fork, whisk egg mixture into chocolate mixture, mixing until slightly moistened. Using hands, knead dough slightly to incorporate all ingredients and shape dough into a ball. Wrap dough ball in plastic wrap and refrigerate for 30-60 minutes, or freeze for 15-20 minutes.

Once dough has chilled, use a tablespoon and scoop and shape dough into tablespoon-size balls. Place powdered sugar in a small bowl and roll each ball of dough in powdered sugar to coat. Place powdered sugar covered dough balls on prepared baking sheets 1 to 2-inches apart.

Bake until just set but still slightly soft and undercooked, about 5-8 minutes. Let cookies cool on baking sheets for 5 minutes, then transfer to wire racks to cool completely. Serve warm.