01 - Preheat oven to 350°F and line a 12-cup muffin tin with paper liners.
02 - Whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl.
03 - In a separate bowl, whisk eggs, milk, vegetable oil, vanilla extract, and peppermint extract until combined.
04 - Pour wet ingredients into dry mixture and mix until just incorporated.
05 - Stir in boiling water carefully; batter will be thin.
06 - Divide batter evenly among liners, filling each about two-thirds full.
07 - Bake 16 to 18 minutes or until a toothpick inserted comes out clean.
08 - Allow to cool in pan for 5 minutes, then transfer to wire rack to cool completely.
09 - Beat butter until creamy; gradually add powdered sugar, milk, peppermint extract, and salt. Beat until light and fluffy.
10 - Pipe or spread buttercream over cooled cupcakes and sprinkle with crushed candy canes.