01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Whisk together flour, cocoa powder, espresso powder, baking powder, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar with electric mixer until light and fluffy, about 2-3 minutes.
04 - Beat in egg and vanilla extract until well combined.
05 - Mix in dry ingredients on low speed, alternating with milk, until just incorporated. Do not overmix.
06 - Scoop tablespoon-sized portions, roll into balls, and coat each ball in extra sugar.
07 - Place cookies 2 inches apart on prepared baking sheets. Flatten slightly with hand or glass bottom.
08 - Bake for 10-12 minutes, until edges are set but centers remain soft.
09 - Cool on baking sheet for 5 minutes, then transfer to wire rack to cool completely.