01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until well blended.
03 - In a separate medium bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until smooth and uniform.
04 - Pour the wet mixture into the bowl with dry ingredients. Mix gently until just combined, avoiding overmixing to maintain muffin tenderness.
05 - Fold in the semi-sweet chocolate chips, mini marshmallows, and crushed peppermint candies until evenly distributed.
06 - Spoon the batter evenly into each muffin cup, filling each about three-quarters full. Garnish with additional chocolate chips, mini marshmallows, and crushed peppermint candies as desired.
07 - Transfer the muffin tin to the oven and bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
08 - Allow muffins to cool in the tin for 5 minutes, then carefully transfer them to a wire rack to cool completely before serving.