Preheat oven to 350°F (175°C). Line a standard muffin pan with paper liners.
Whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
In a large bowl, mix the sugar, butter, espresso, vanilla, and egg until thoroughly combined. Add the flour mixture in 3 or 4 portions, mixing just until combined. Stir in the chocolate chips.
Transfer the batter to the prepared muffin pan, using about 1/4 cup batter per muffin.
Bake 20-25 minutes or until a pick inserted into the center comes out with clean. Keep in mind that you may get chocolate on the pick from the chips.