01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
03 - In a large bowl, whisk eggs and sugar until light and fluffy. Incorporate oil, milk, vanilla extract, and peppermint extract until combined.
04 - Add dry ingredients to wet ingredients in two additions, stirring gently after each to avoid overmixing.
05 - Distribute batter evenly among liners, filling each about two-thirds full.
06 - Bake for 16 to 18 minutes, or until a toothpick inserted in the center comes out clean. Let cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
07 - Beat butter until creamy. Gradually incorporate powdered sugar, then milk, peppermint extract, and salt. Continue beating until fluffy and smooth.
08 - Pipe or spread frosting on cooled cupcakes. Garnish with crushed peppermint candies if desired.