01 - Preheat oven to 350°F and 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 with granulated 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 optional chocolate chips.
07 - Spoon batter evenly into muffin cavities, filling 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 - Let cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.