01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt.
03 - In a medium bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until smooth.
04 - Pour wet mixture into dry ingredients and gently mix with a spatula until just combined. Avoid overmixing.
05 - Carefully fold in semi-sweet chocolate chips, mini marshmallows if using, and crushed peppermint candies until distributed.
06 - Spoon batter evenly into paper-lined muffin cups, filling each about three-quarters full.
07 - Sprinkle additional chocolate chips, marshmallows, and crushed peppermint candies on top of each muffin.
08 - Bake 18 to 20 minutes, or until a toothpick inserted into the center emerges with a few moist crumbs.
09 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.