01 - Preheat the oven to 350°F and 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 thoroughly blended.
03 - In a separate bowl, beat eggs with whole milk, vegetable oil, peppermint extract, and vanilla extract until smooth.
04 - Pour the wet mixture into the dry mixture and gently stir with a spatula until just combined. Do not overmix.
05 - Carefully fold in the chocolate chips, mini marshmallows (if using), and crushed peppermint candies until evenly distributed.
06 - Divide the batter evenly among the muffin cups, filling each about three-quarters full.
07 - Sprinkle additional chocolate chips, mini marshmallows, and crushed peppermint candies over each portion as desired.
08 - Bake on the center rack for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin emerges with a few moist crumbs attached.
09 - Allow muffins to rest in the pan for 5 minutes before transferring to a wire rack to cool completely.