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 well blended.
04 - Pour wet mixture into dry ingredients and stir gently with a spatula until just combined, avoiding overmixing.
05 - Fold in chocolate chips and nuts if desired.
06 - Divide batter evenly into muffin cups, filling each about 3/4 full.
07 - Bake for 18 to 20 minutes until a toothpick inserted in the center comes out with a few moist crumbs.
08 - Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.