01 - Set oven to 350°F. Line a standard 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, thoroughly whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
03 - In a separate medium bowl, whisk eggs, milk, vegetable oil, peppermint extract, and vanilla extract until uniformly blended.
04 - Pour wet mixture over dry ingredients and gently stir until batter just comes together. Avoid overmixing.
05 - Fold chocolate chips, mini marshmallows, and crushed peppermint candies into batter.
06 - Spoon batter evenly into prepared muffin cups, filling each about three-quarters full.
07 - Sprinkle extra chocolate chips, mini marshmallows, and additional crushed peppermint candies atop each muffin, if desired.
08 - Bake in preheated oven for 18 to 20 minutes, or until a toothpick inserted in the center yields a few moist crumbs.
09 - Let muffins rest in tin for 5 minutes. Transfer to wire rack until completely cooled.