01 - Preheat the oven to 350°F (175°C). Line a mini muffin tin with paper liners or grease thoroughly.
02 - In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt until evenly blended.
03 - In a large bowl, whisk melted butter with granulated and brown sugars until smooth and well incorporated.
04 - Add eggs one at a time into the wet mixture, beating well after each addition, then stir in vanilla extract.
05 - Fold the dry ingredients into the wet mixture gently until just combined, avoiding overmixing.
06 - Carefully fold in chopped nuts and optional chocolate chips until evenly distributed.
07 - Spoon batter evenly into prepared muffin cavities, filling each about three-quarters full.
08 - Bake for 15 to 20 minutes until tops are set and a toothpick inserted in the center comes out with moist crumbs.
09 - Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before serving.