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, cocoa powder, granulated sugar, baking powder, baking soda, and salt until evenly incorporated.
03 - In a separate bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until fully blended.
04 - Pour blended wet ingredients into the bowl with dry ingredients and stir gently until just combined, avoiding overmixing.
05 - Fold in semi-sweet chocolate chips, mini marshmallows if using, and crushed peppermint candies, distributing evenly throughout the batter.
06 - Divide the batter evenly between muffin liners, filling each about three-quarters full.
07 - Scatter additional chocolate chips, mini marshmallows, and crushed peppermint candies on top of each filled muffin cup.
08 - Bake for 18 to 20 minutes, or until a toothpick inserted in the center emerges with a few moist crumbs attached.
09 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.