Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. In a large bowl, cream together the butter and sugar using an electric mixer until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract. Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, and mix until just combined.

Divide the batter evenly among the prepared cupcake liners, filling each about two-thirds full. Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Remove from the oven and let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

In a large bowl, beat the softened butter with an electric mixer until creamy. Gradually add the powdered sugar, one cup at a time, mixing well after each addition. Add the heavy cream and peppermint extract, and beat until smooth and fluffy. If the frosting is too thick, add a little more heavy cream; if it's too thin, add more powdered sugar.

Once the cupcakes are completely cool, frost them using a piping bag or a spatula. Garnish each cupcake with crushed peppermint candies for an extra festive touch. Serve immediately or store as directed.

Store the cupcakes in an airtight container at room temperature for up to three days. For longer storage, refrigerate or freeze them, and bring them to room temperature before serving for best texture.