01 - Set oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until uniform.
03 - In a separate bowl, whisk the eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until fully incorporated.
04 - Pour the wet ingredients into the dry mixture. Mix gently with a spatula until just combined, being careful not to overmix.
05 - Gently fold chocolate chips, mini marshmallows, and crushed peppermint candies into the batter.
06 - Divide batter evenly among muffin cups, filling each approximately three-quarters full. Scatter additional chocolate chips, marshmallows, and peppermint candies over each for garnish.
07 - Bake for 18 to 20 minutes, or until a toothpick inserted into a muffin emerges with a few moist crumbs.
08 - Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.