01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large bowl, vigorously whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt until evenly combined.
03 - In a separate medium bowl, whisk together eggs, milk, vegetable oil, peppermint extract, and vanilla extract until smooth and fully blended.
04 - Add the wet mixture into the dry mixture and gently fold together using a spatula until just combined. Avoid overmixing the batter.
05 - Fold in chocolate chips, mini marshmallows if using, and crushed peppermint candies until evenly distributed throughout the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Top with extra chocolate chips, marshmallows, and peppermint candies if desired.
07 - Bake for 18 to 20 minutes, or until a toothpick inserted in the center of a muffin comes out with just a few moist crumbs.
08 - Allow muffins to cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely.