01 - Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt until fully blended.
03 - In a separate bowl, whisk eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until the mixture is smooth and homogenous.
04 - Pour wet ingredients into dry ingredients and mix gently with a spatula until just combined, taking care not to overmix.
05 - Gently fold in chocolate chips, mini marshmallows (if using), and crushed peppermint candies until evenly distributed.
06 - Divide batter evenly among muffin cups, filling each approximately three-quarters full.
07 - Sprinkle additional chocolate chips, mini marshmallows, and peppermint candies over the tops of each muffin.
08 - Bake in preheated oven for 18 to 20 minutes, or until a toothpick inserted in the center emerges with a few moist crumbs attached.
09 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.