Preheat the oven to 350 degrees F.
Whisk together boiling water and cocoa. Cover and set aside. With a mixer, cream together the butter, sugar, salt and vanilla extract. Add the eggs, 1 at a time, and continue creaming until light and fluffy. Sift together the flour, baking soda and baking powder and add this mixture, alternately with the buttermilk. Add cocoa and water mixture and mix until thoroughly combined. Scoop into cupcake pans and bake for 20 to 25 minutes.
Let cool completely before decorating. Dip tops of cooled cupcakes in the warm ganache and let set. Pipe more ganache on top for decoration. To fill, put buttercream in a pastry bag with a small round tip. Stick tip into the center of each cupcake and pipe in filling. Be careful not to pipe too much or you will break the cupcake. Serve.