01 - Preheat oven to 350°F. Line a mini muffin tin with paper liners or grease thoroughly.
02 - Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl.
03 - In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth.
04 - Add eggs one at a time, beating well after each addition, then stir in vanilla extract.
05 - Fold dry ingredients into wet mixture until just combined, avoiding overmixing.
06 - Gently fold in chopped nuts and chocolate chips if using.
07 - Spoon batter evenly into mini muffin cavities, filling each about three-quarters full.
08 - Bake for 15 to 20 minutes until tops are set and a toothpick inserted comes out with moist crumbs.
09 - Cool in pan for 10 minutes before transferring to a wire rack to cool completely.