01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease.
02 - In a large mixing bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, beat eggs, then whisk in milk, vegetable oil, vanilla extract, and Greek yogurt until smooth and uniform.
04 - Pour wet mixture into dry ingredients and gently fold together just until combined. Avoid overmixing.
05 - Fold in chocolate chips and optional chopped nuts until evenly distributed.
06 - Divide batter evenly among muffin cups, filling each about three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted in the center emerges with a few moist crumbs.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.