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.