Make the cupcakes: Preheat your oven to 325°F. In a medium bowl, pour the hot coffee over the chopped coffee. Let stand for a minute, then stir until the chocolate has melted completely and the mixture is smooth.
In a stand mixer, or using a hand-held electric mixer, beat your egg until frothy, about 30 seconds. Add the sour cream, oil, and vanilla. Mix.
Add your chocolate and beat until all ingredients are well combined.
In a large bowl, whisk together your flour, sugar, baking powder, baking soda, salt, and cocoa powder. Add it to your wet ingredients and mix just until combined.
Divide the batter evenly among a paper-lined muffin tin. Bake until a toothpick inserted in the center of one comes out clean, 18 to 20 minutes. Let them cool in the pan set on top of a wire rack for 15 minutes before removing to cool completely.
Make the cream filling: In a stand mixer, beat the butter until it is creamy and smooth. Slowly add the confectioner's sugar, one cup at a time, and beat on medium-high until fluffy. Add the cream and vanilla and continue to beat until the mixture is light and fluffy. Add more cream, one tablespoon at a time, if necessary to achieve a creamy, light consistency.
Make the chocolate glaze: In a small saucepan, combine all ingredients and heat over medium until smooth and well-combined. Remove from heat and let the mixture cool for 30 minutes before spreading on top of the cupcakes.
Assemble the cupcakes:
When the cupcakes have completely cooled, use a sharp knife to carefully remove the centers of each cupcake, being careful not to cut through to the bottom of the cupcake.
Now it's time to fit your pastry bag, or a gallon-size resealable plastic bag with the bottom corner snipped, with a medium-sized round opening tip (any one will do- the design does not matter. This will allow us to pipe the buttercream easily into the center of each cupcake.
To fill the bag with buttercream, fold over the top edges outwardly, then use a spatula to scoop the buttercream inside. Unfold the edges of the bag and squeeze the buttercream downwards with your hands to reach the tip.
Twist the bag tightly around the bulge of buttercream so that no buttercream squeezes out the top of the bag while you’re piping.
Squeeze the buttercream into the hollow center of each cupcake, filling each with about 2 tablespoons of buttercream.
Make the ganache (the shiny chocolate glaze for the cupcakes). Microwave your heavy cream in a microwave safe measuring cup or bowl for 1 minute, or until it begins to bubble. Pour it over your chopped chocolate in a small bowl. Let it stand for a minute and then stir until smooth and shiny. Let it cool to room temperature before spreading on the cupcakes.
When you're ready, spoon a tablespoon of ganache onto the top of each cupcake and smooth to the edges using a butter knife. Don't worry if the ganache falls over the sides.
Change the tip on your pastry bag to a small round opening (the smallest you have) in order to create that signature squiggle that Hostess pipes along each of their cupcakes. You could also use a plastic bag and snip the corner - but note that the squiggly line might be a little more “flat” than one made with a pastry bag. No one minds.) Practice piping the squiggly line on a plate a few times until you get the hang of it. Refill your bag with more buttercream if necessary.
Be confident and use even, gentle pressure when piping. Repeat on remaining cupcakes.