01 - Set 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, granulated sugar, baking powder, baking soda, and salt until evenly blended.
03 - In a separate bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until smooth.
04 - Pour the wet mixture into the dry ingredients. Fold the mixture gently until just incorporated; avoid overmixing for a tender texture.
05 - Fold in semi-sweet chocolate chips, mini marshmallows if using, and crushed peppermint candies until evenly distributed.
06 - Divide batter evenly among muffin liners, filling each about three-quarters full.
07 - Sprinkle additional chocolate chips, mini marshmallows, and crushed peppermint candies over each filled cup for topping.
08 - Transfer pan to oven and bake for 18 to 20 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
09 - Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.