Preheat oven to 320°F (160°C). Line muffin tins paper cupcake liners.
Whisk together cocoa powder and hot coffee in a medium-sized bowl and set aside.
In a large mixing bowl, combine flour, sugar, baking soda, baking powder and salt. Add egg, buttermilk, melted butter and vanilla extract and beat on medium speed for 3 minutes. Scrape sides of bowl as needed. Stir in the coffee and cocoa mixture. The cupcake batter will be very runny.
Divide the batter into 16 cupcake liners, about ¼ cup of batter per cupcake. Make sure cupcake liners are not more than two thirds full. Bake cupcakes for 15-17 minutes. Check for doneness with a wooden toothpick. Toothpick should come out clean.
Remove cupcakes from the oven and allow them to cool for 10 minutes in the pan on a wire rack, then transfer cupcakes to a wire rack to cool completely.
Beat the butter and salt until creamy. Sift powdered sugar and cocoa powder together in a bowl. Add to the butter, along with the sour cream, whipping cream and vanilla. Beat for 1-2 minutes on high speed until the buttercream is creamy and fluffy. Scrape bottom and sides of mixing bowl with a rubber spatula as needed. If the frosting is still too stiff, beat in more cream, one teaspoon at a time, until the buttercream has reached the desired consistency.
To frost the cupcakes: Fill a piping bag fitted with a large star tip with the frosting. Pipe swirls of frosting onto the cooled cupcakes, beginning at an outside edge and ending in the center. Top with colored sprinkles, chocolate sprinkles or mini M&M’s if desired.
Stored tightly covered in the refrigerator, cupcakes will stay fresh for up to five days. Allow the cupcakes to stand at room temperature for 20 minutes before serving.