01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease with nonstick spray.
02 - In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
03 - In a separate bowl, beat eggs, then whisk in milk, vegetable oil, and vanilla extract until fully blended.
04 - Pour wet ingredients into dry ingredients and fold gently with a spatula until just combined; avoid overmixing.
05 - Fold in semi-sweet chocolate chips and chopped nuts if using.
06 - Divide batter evenly among muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 20 minutes until a toothpick inserted into the center comes out with a few moist crumbs.
08 - Let muffins cool in the tin for five minutes, then transfer to a wire rack to cool completely.