01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk flour, cocoa powder, sugar, baking powder, baking soda, and salt until evenly blended.
03 - In a separate bowl, whisk together eggs, milk, vegetable oil, peppermint extract, and vanilla extract until fully combined.
04 - Pour wet ingredients into dry ingredients and gently stir until just blended. Do not overmix to ensure tender muffins.
05 - Gently fold in chocolate chips, mini marshmallows (if using), and crushed peppermint candies.
06 - Evenly distribute the batter among muffin cups, filling each about 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 into the center emerges with a few moist crumbs attached.
09 - Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack until completely cooled.