01 - Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until uniformly combined.
03 - In a separate medium bowl, vigorously whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until fully integrated.
04 - Pour wet ingredients over dry ingredients. Gently fold together just until combined, avoiding overmixing.
05 - Fold in chocolate chips, mini marshmallows (if using), and crushed peppermint candies until evenly dispersed.
06 - Divide batter equally among muffin cups, filling each approximately three-quarters full.
07 - Sprinkle additional chocolate chips, mini marshmallows, and crushed peppermint candies over each muffin.
08 - Bake for 18 to 20 minutes, or until a toothpick inserted in the center emerges with only a few moist crumbs attached.
09 - Let cool in the pan for 5 minutes, then transfer muffins to a wire rack to cool completely.