01 - Preheat the oven to 350°F. Line 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 well blended.
03 - In a separate bowl, whisk the eggs, whole milk, vegetable oil, peppermint extract, and vanilla extract until the mixture is smooth and fully combined.
04 - Pour the blended wet ingredients into the dry ingredients. Stir gently with a spatula or whisk just until incorporated. Avoid overmixing to ensure a tender crumb.
05 - Carefully fold the chocolate chips, mini marshmallows if using, and crushed peppermint candies into the batter until evenly distributed.
06 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Top each muffin with additional chocolate chips, marshmallows, and crushed peppermint candies as desired.
07 - Place the pan in the preheated oven and bake for 18 to 20 minutes. Muffins are ready when a toothpick inserted in the center emerges with a few moist crumbs attached.
08 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.