01 - Preheat the oven to 350°F. Line a standard 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt until evenly blended.
03 - In a medium bowl, whisk the eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until thoroughly incorporated.
04 - Pour the wet mixture over the dry ingredients. Gently fold until just combined, taking care not to overmix the batter.
05 - Fold in the semi-sweet chocolate chips, mini marshmallows, and crushed peppermint candies until evenly distributed throughout the batter.
06 - Divide the batter evenly among the muffin cups, filling each approximately three-quarters full.
07 - Sprinkle the tops of each muffin with extra chocolate chips, mini marshmallows, and additional crushed peppermint candies for garnish as desired.
08 - Transfer the muffin tin to the preheated oven and bake for 18 to 20 minutes, or until a toothpick inserted into the center emerges with a few moist crumbs attached.
09 - Allow the muffins to cool in the pan for 5 minutes. Carefully transfer them to a wire rack to cool completely before serving.