01 - Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt until well blended.
03 - In a medium bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until thoroughly combined.
04 - Pour wet mixture into dry ingredients and mix gently until just combined. Avoid overmixing.
05 - Carefully fold in semi-sweet chocolate chips, mini marshmallows, and crushed peppermint candies until evenly distributed.
06 - Divide batter evenly among prepared muffin cups, filling each about three-quarters full.
07 - Top each filled cup with additional chocolate chips, mini marshmallows, and crushed peppermint candies as desired.
08 - Bake for 18 to 20 minutes or until a toothpick inserted into the center emerges with a few moist crumbs attached.
09 - Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack and cool completely before serving.