01 - Preheat oven to 350°F and line a 12-cup muffin tin with paper liners.
02 - Whisk together flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar in a large bowl until light and fluffy. Add eggs one at a time, then mix in vanilla and peppermint extracts.
04 - Add dry ingredients to the butter mixture in three parts, alternating with milk and sour cream. Start and finish with dry ingredients, mixing until just combined.
05 - Divide batter evenly among the prepared cupcake liners.
06 - Use a toothpick to drag through each cupcake to form a peppermint swirl pattern.
07 - Bake for 18 to 20 minutes or until a toothpick inserted into the center comes out clean. Cool completely on a wire rack.
08 - Beat butter until creamy, then gradually add powdered sugar, milk, and peppermint extract. Beat until fluffy.
09 - Optionally, add red gel food coloring and gently swirl with a spatula to create a decorative effect without overmixing.
10 - Pipe frosting onto cooled cupcakes and sprinkle with crushed peppermint candies.