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 until evenly 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 the bowl with dry ingredients. Stir gently until just incorporated; avoid overmixing to maintain a tender crumb.
05 - Gently fold semi-sweet chocolate chips, mini marshmallows, and crushed peppermint candies into the batter.
06 - Distribute batter evenly into lined muffin cups, filling each approximately three-quarters full. Sprinkle with additional chocolate chips, mini marshmallows, and peppermint candy as desired.
07 - Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin emerges with a few moist crumbs attached.
08 - Allow muffins to rest in the tray for 5 minutes, then transfer to a wire rack to cool completely before serving.