01 - Preheat your oven to 350°F (175°C). Line a 12-cup muffin pan with cupcake liners.
02 - In a large bowl, sift together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, baking soda, and salt.
03 - In a separate bowl, whisk together the large eggs, whole milk, vegetable oil, pure vanilla extract, instant espresso powder, and peppermint extract until well combined.
04 - Add the wet ingredients to the dry ingredients and mix until just combined. Do not overmix.
05 - Gently stir in the boiling water until the batter is smooth. The batter will be thin.
06 - Divide the batter evenly among the prepared cupcake liners, filling each approximately 2/3 full.
07 - Bake for 16–18 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
08 - For the peppermint buttercream, beat the softened unsalted butter with an electric mixer on medium speed until it is creamy, approximately 2 minutes.
09 - Gradually add the sifted powdered sugar, mixing on low speed until incorporated. Add the heavy cream, peppermint extract, and a pinch of salt. Beat on high speed until the mixture is light and fluffy, about 3 minutes.
10 - Once the cupcakes have completely cooled, pipe or spread the peppermint buttercream onto the tops.
11 - Decorate the frosted cupcakes with mini chocolate chips, crushed peppermint candies, and a drizzle of chocolate syrup, if desired.