Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. This recipe yields about 15 cupcakes, so line a second muffin pan with 3 cupcake liners or bake in batches.

Whisk the flour, cocoa powder, baking soda, baking powder, and salt together in a large bowl. Set aside. Whisk the oil, sugar, egg, vanilla, and buttermilk together until combined. Pour the wet ingredients into the dry ingredients, add the hot coffee/water, and whisk until the batter is completely combined. Batter is thin.

Pour/spoon the batter into the liners, filling only 2/3 full to avoid spilling over the sides. You should have enough batter for 15 cupcakes.

Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean.

Allow cupcakes to cool in the pan for 10 minutes, and then transfer to a wire rack to cool completely. Cupcakes must be completely cooled—and even chilled for 20–30 minutes in the refrigerator if they’re particularly sticky on top—before assembling.

Place finely chopped chocolate in a medium heatproof bowl. Heat the cream in a small saucepan over medium heat until it begins to gently simmer. (Do not let it come to a rapid boil—that’s too hot!) Pour over chocolate, then let it sit for 2–3 minutes to gently soften the chocolate. With a metal spoon or small rubber spatula, very slowly stir until chocolate has melted and mixture is smooth. Ganache is thin. The finer you chopped the chocolate, the quicker it will melt with the cream. If it’s not melting, do not microwave it. If needed, see Troubleshooting Chocolate Ganache. Once ganache mixture is smooth, transfer it to the refrigerator and let it chill for at least 30 minutes to thicken before spreading on cooled and filled cupcakes.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 3 full minutes. Add up to 1/4 cup (30g) more confectioners’ sugar if frosting is too thin or another 1 Tablespoon (15ml) of cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. (I add a very tiny pinch of salt.) Frosting should be extra fluffy.

Using a sharp knife, cut a circle into the center of the cooled cupcakes to create a little pocket about 1 inch deep. The piece you removed will be sort of cone-shaped. Spoon or pipe whipped buttercream inside each carved-out cupcake—use however much frosting will fit. (I use either a teaspoon measuring spoon or Wilton piping tip #12 to fill cupcakes. Usually you can fit between 1–2 teaspoons of frosting in each.) Slice/tear off the pointy end of the “cone” piece of cupcake you removed, and gently press the round piece back on top of the filling. See my How to Fill Cupcakes post if you need extra visuals or help with this step.

With a knife or small icing spatula, spread the thickened chocolate ganache on top of each cupcake. Spread on a thick layer, so it sets as a thick fudge-like topping.

Fill a piping bag with the remaining vanilla buttercream. Pipe a looping swirl, or any design you prefer, on top of the ganache-covered cupcake. (No need to chill ganache-topped cupcakes before doing this.) I use Wilton piping tip #6 for piping.

Serve immediately or store covered at room temperature or in the refrigerator for up to 1 day before serving. Cover and store leftover cupcakes in the refrigerator for up to 3 days. I recommend a cupcake carrier for storing and transporting decorated cupcakes.