01 - Preheat oven to 350°F. Grease a 24-cup mini muffin pan or line with mini paper liners.
02 - Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl.
03 - In a large bowl, whisk melted butter and sugar until fully incorporated. Add eggs and vanilla extract; whisk until smooth.
04 - Gradually fold dry mixture into wet ingredients until just combined, avoiding overmixing.
05 - Stir in semi-sweet chocolate chips and chopped nuts, if using.
06 - Divide batter evenly among mini muffin cups, filling each about three quarters full.
07 - Bake for 12 to 15 minutes, or until tops are set and a toothpick inserted into the center comes out with moist crumbs.
08 - Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.