For the cupcakes:
¾ cup unsweetened Dutch-process cocoa powder
¾ cup hot water
3 cups all-purpose flour
1 tsp. baking soda
1 tsp. baking powder
1¼ tsp. coarse salt
1½ cups (3 sticks) unsalted butter
2¼ cups sugar
4 large eggs, at room temperature
4 tsp. vanilla extract
1 cup sour cream, at room temperature
For the ganache filling:
8 oz. bittersweet chocolate, finely chopped
1 cup plus 2 tbsp. heavy cream
4 tbsp. unsalted butter, cut into 4 pieces, at room temperature
For the frosting:
14 oz. bittersweet chocolate, finely chopped
9 oz. cream cheese, at room temperature
9 tbsp. unsalted butter, at room temperature
3 cups confectioners’ sugar, sifted
6 tbsp. unsweetened Dutch-process cocoa powder
Pinch of salt
1 cup plus 2 tbsp. sour cream
For the chocolate curls:
Large block of high-quality semi- or bittersweet chocolate
To make the cupcakes, preheat the oven to 350˚ F. Line standard cupcake pans with paper liners. In a small bowl, whisk together the cocoa powder and hot water until smooth. In a medium bowl, whisk together the flour, baking soda, baking powder, and salt; set aside.
In a medium saucepan, combine the butter and the sugar over medium heat. Heat, stirring occasionally to combine, until the butter is melted. Remove the mixture from the heat and transfer to the bowl of an electric mixer fitted with a paddle attachment. Beat on medium-low speed, 4-5 minutes, until the mixture is cooled. Mix in the eggs, one at a time, scraping down the sides of the bowl as needed and beating well after each addition. Mix in the vanilla and then the cocoa mixture and beat until incorporated. With the mixer on low speed add in the dry ingredients in two batches, alternating with the sour cream, beating just until combined.
Divide the batter between the prepared cupcake liners, filling them about ¾ of the way full. Bake 18-20 minutes or until a toothpick inserted in the center comes out clean, rotating the pans halfway through baking. Allow the cupcakes to cool in the pan 5-10 minutes, then transfer to a wire rack to cool completely.
To make the ganache, place the chopped chocolate in a medium bowl. Heat the cream in a small saucepan and bring to a boil. Once boiling, remove from the heat and pour over the chopped chocolate. Let stand about 1 minute. With a whisk, gently stir the chocolate and cream together in small circles. When the ganache has become smooth, whisk in the butter 1 piece at a time until incorporated. Transfer the bowl to the freezer or refrigerator to chill and thicken the ganache a bit. Whisk every 5-10 minutes to help it cool evenly. Once the mixture is slightly thickened and no longer runny, you are ready to fill the cupcakes.
To assemble the cupcakes, cut a cone out of the center of each cupcake with a small paring knife. Cut off the pointed end of the cone and discard (or eat) so you are left with a cap of cake to cover the filling. Drop a spoonful of ganache into the center of each cupcake and recover the hole with the cap.
To make the frosting, melt the chocolate in a heat-proof bowl set over a pot of simmering water. Set aside to cool until just barely warm. In the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese and butter on medium-high speed until pale and fluffy, about 3-4 minutes. Gradually mix in the confectioners’ sugar, cocoa powder and salt. Beat in the melted and cooled chocolate and then the sour cream. Continue beating until the mixture is smooth and well blended. Frost cupcakes immediately.
To make the chocolate curls, warm the block of chocolate just slightly (heat in 5-second intervals in the microwave – be careful not to melt). Use a vegetable peeler to create chocolate curls. Top cupcakes with curls immediately. Reheat chocolate as needed to keep it workable.