01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together all-purpose flour, cocoa powder, sugar, baking powder, baking soda, and salt until thoroughly blended.
03 - In a separate bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until the mixture is smooth and uniform.
04 - Pour the wet ingredient blend into the dry ingredients. Mix gently with a spatula until just combined; avoid overmixing to maintain a tender crumb.
05 - Fold in the semi-sweet chocolate chips, mini marshmallows, and crushed peppermint candies until evenly distributed throughout the batter.
06 - Divide the batter evenly among the muffin cups, filling each approximately three-quarters full.
07 - Sprinkle additional chocolate chips, mini marshmallows, and crushed peppermint candies on top of each muffin as desired.
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 rest in the pan for 5 minutes before transferring to a wire rack to cool fully.