For the cupcakes:
8 tablespoons unsalted butter, cut into 4 pieces
2 ounces bittersweet chocolate, chopped
½ cup Dutch-process cocoa
¾ cup all-purpose flour
½ teaspoon baking soda
¾ teaspoon baking powder
2 large eggs
¾ cup granulated sugar
1 teaspoon vanilla
½ teaspoon salt
½ cup sour cream
For the mousse:
2 tablespoons Dutch-process cocoa powder
5 tablespoons hot water
7 ounces bittersweet chocolate, finely chopped
1½ cups heavy cream
1 tablespoon granulated sugar
⅛ teaspoon salt
Start by making the cupcakes. Place oven rack in lower-middle position and heat oven to 350º. Butter and flour a 12 cup muffin pan.
In a medium bowl, combine butter, chocolate and cocoa. Set over a small saucepan with simmering water and heat until melted. Remove from heat and whisk until smooth and thoroughly combined. Set aside to cool until just warm to the touch.
In a small bowl, whisk together flour, baking soda and baking powder. Set aside. In the bowl of a stand mixer fitted with the whisk attachment, whisk eggs. Add sugar, vanilla and salt and mix on medium-low speed until combined. Add cooled chocolate mixture and continue mixing until chocolate is incorporated. Sift in one-third of the flour mixture and continue mixing, scraping down the sides of bowl as needed. Mix in sour cream, then sift in remaining flour mixture. Continue mixing and scraping the bowl until everything is combined. The batter will be thick.
Divide batter evenly into muffin pan cups and bake for about 18-20 minutes, or until a skewer inserted in the center comes out clean. Cool completely on a wire rack.
While the cupcakes are cooling, make the mousse. In a small bowl, combine cocoa powder and water; set aside. Place chocolate in a medium bowl. Set over a small saucepan with simmering water, stirring until melted and smooth. Set aside to cool slightly, about 5 minutes.
Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment, whip cream, sugar and salt at medium speed until slightly thickened, about 30 seconds. Increase speed to high and continue whipping until soft peaks form. Whisk the cocoa mixture into the chocolate until smooth. Fold one-third of the whipped cream into the chocolate mixture to lighten it. Using a rubber spatula, fold in remaining cream until combined and no white streaks remain.
To make the cauldron cakes, place a sheet of wax paper on a small cutting board or flat plate. Set aside.
Remove centers of cupcakes using the cone method. Melt about 6 ounces of chocolate in a shallow bowl. Dip the tops of the cupcakes and set aside. Place remaining chocolate in a piping bag. Pipe cauldron handles onto wax paper and place in freezer for about 10 minutes. Meanwhile, pipe or spoon mousse into cupcakes. Once the chocolate handles are cooled completely, remove from wax paper and place on top of each cupcake.