01 - Preheat oven to 350°F. Line all wells of a 12-cup muffin tin with paper liners.
02 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt until thoroughly blended.
03 - In a medium bowl, whisk the eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until the mixture is smooth and well combined.
04 - Add the wet ingredients to the dry ingredients and gently mix until just combined; avoid overmixing to preserve a tender crumb.
05 - Fold in the chocolate chips, mini marshmallows if using, and crushed peppermint candies until distributed evenly throughout the batter.
06 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Garnish each with extra chocolate chips, mini marshmallows, and additional crushed peppermint candies as desired.
07 - Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out mostly clean with only a few moist crumbs attached.
08 - Allow muffins to cool in the pan for five minutes. Transfer to a wire rack and leave to cool completely before serving or storing.