01 - Preheat oven to 350°F. Grease a 24-cup mini muffin pan or line with mini paper liners.
02 - In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt until evenly mixed.
03 - In a large bowl, whisk melted butter and sugar until well combined. Add eggs and vanilla extract; whisk until smooth.
04 - Gradually fold the dry ingredients into the wet mixture until just combined, taking care not to overmix.
05 - Gently stir in semi-sweet chocolate chips and chopped nuts if using.
06 - Divide the 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 yields moist crumbs.
08 - Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.