01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Whisk together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt in a large mixing bowl.
03 - In a separate bowl, whisk together eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until homogenous.
04 - Pour the wet mixture into the bowl with the dry ingredients. Gently stir until just combined, taking care not to overmix.
05 - Fold in semi-sweet chocolate chips, mini marshmallows if desired, and crushed peppermint candies until evenly distributed.
06 - Spoon the batter evenly among the prepared muffin cups, filling each about three-quarters full.
07 - Sprinkle additional chocolate chips, mini marshmallows, and more crushed peppermint candies over the tops of the muffins as desired.
08 - Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs attached.
09 - Let muffins cool in the pan for 5 minutes before transferring to a wire rack to cool completely.