Step 1

For the cake: Preheat the oven to 350 degrees F. Line two 12-cup muffin tins with paper liners.

Step 2

Whisk together the cake flour, cocoa powder, granulated sugar, baking powder, baking soda and salt in the bowl of a stand mixer. Whisk together the buttermilk, coffee, oil, eggs and vanilla in a separate bowl.

Step 3

Pour half of the wet ingredients into the dry ingredients and beat on low speed just until combined. Scrape down the bowl then add the remaining wet ingredients and beat to combine. Scrape down the bowl and paddle. Increase the speed to medium and beat for a full minute.

Step 4

Fill the prepared cupcake tins 2/3 of the way full. Bake until a toothpick inserted in the center of each comes out clean and the top springs back when lightly pressed, 15 to 18 minutes.

Step 5

For the frosting: Melt the chocolate in a medium heatproof bowl set over simmering water in a medium saucepan. Remove from the simmering water and let it cool until room temperature, about 30 minutes.

Step 6

Beat the butter and salt in a large bowl with an electric mixer on high speed until light and fluffy, about 2 minutes. Reduce the speed to low, drizzle in the melted chocolate and beat until completely blended. Gradually beat in the confectioners' sugar, cocoa powder and vanilla. Increase the speed to high and beat until light and fluffy, about 2 minutes.

Step 7

Transfer the frosting to a pastry bag fitted with a large star tip. Decoratively pipe the frosting onto the cupcakes.