These dark chocolate cupcakes are rich, moist, and decadent... even before being filled with chocolate hazelnut Nutella spread! Frost them with a triple berry Swiss meringue buttercream and decorate with fresh raspberries, blueberries, and blackberries.
Preheat oven to 350 degrees and line a muffin tin with paper liners. This recipe makes 12-14 cupcakes.
In a large bowl, whisk together the flour, sugars, cocoa powder, baking soda, and salt until combined.
Add the egg, egg yolk, buttermilk, oil, and vanilla and mix well. Pour in the hot coffee and mix until just combined.
Divide the batter evenly among the muffin tins, filling each one about 3/4 of the way full. Bake at 350 degrees for 15-17 minutes, or until a toothpick inserted into the center of the cupcakes comes out with just a few moist crumbs attached. Let cool in the tins for about 5 minutes, then transfer to a wire rack to cool completely.
Combine the berries and 1 tablespoon sugar in a small saucepan over medium-low heat, stirring occasionally until juices are released and sugar liquifies.
Stirring frequently, let the mixture simmer for about 10 minutes, until reduced. Use a wooden spoon to help break down the berries as they cook.
Remove from heat and press the puree through a mesh sieve (over a small bowl) to release the juice. Dispose of the seeds and set aside the juice to cool.
Fill the bottom of a double boiler with 1-2 inches of water (water shouldn’t touch the top), then place the egg whites and 2/3 cup sugar in the top bowl. Whisk to combine, then turn heat to medium-low.
Whisking frequently, heat over simmering water until the mixture registers about 160 degrees Fahrenheit on a candy thermometer. When it's ready, the mixture will feel smooth when rubbed between two fingers (i.e. no sugar granules).
Remove from heat and pour the mixture into the bowl of a stand mixer fitted with the whisk attachment. Beat on high speed for 8-10 minutes until stiff peaks form. The meringue should have cooled to room temperature.
Swap out the whisk for the paddle attachment and reduce speed to low. Add the butter, one tablespoon at a time, mixing until incorporated.
Add the vanilla, then gradually add the berry reduction, a tablespoon at a time, mixing to combine after each addition. Be careful to not overwhelm the buttercream with too much liquid. Yours may be able to handle all of the berry puree, or less, depending on how much juice your berries produced.
Beat on medium speed until the buttercream is smooth and fluffy. It may take a few minutes, but keep mixing until it becomes smooth!
Once the cupcakes are cool, use a cupcake corer (or a small paring knife) to remove the center from each cupcake:
Set the serrated edge of the corer against the center of the dome. Pressing down, twist the corer until the shaft is completely buried. Pull up to remove the core and press down on the top to eject it.
Using a paring knife, slice the cupcake core in half (the top half will act as a lid for the cupcake filling). Spoon Nutella into the center of the cupcake, then replace the lid on top to seal in the filling.
Fill a pastry bag fitted with a large star tip with the buttercream, then frost each cupcake as desired. Arrange berries on top!
*Buttermilk substitute - add 1 1/2 teaspoons white vinegar or lemon juice to a scant 1/2 cup of milk and stir. Let sit for 5 minutes before using.
**Fresh or frozen berries will work here!