For the cupcakes:
½ cup plus 1 tablespoon Dutch-process cocoa powder
½ cup plus 1 tablespoon hot water
2¼ cups all-purpose flour
¾ tsp. baking soda
¾ tsp. baking powder
½ tsp. salt
2 sticks plus 1 tablespoon unsalted butter, at room temperature
1 2/3 cups sugar
3 large eggs, at room temperature
1 tbsp. vanilla extract
¾ cup sour cream
For the filling:
1 cup caramel sauce
For the frosting:
1¼ cup sugar, divided
5 tbsp. water
5 tbsp. heavy cream
Generous pinch of sea salt, such as fleur de sel
5 large egg whites
3¾ sticks (30 tbsp.) unsalted butter, at room temperature
Salted caramels, for garnish, optional
To make the cupcakes, preheat the oven to 350 F. Line cupcake pans with paper liners. In a small bowl, combine the cocoa powder and hot water, whisk until smooth. In another medium bowl combine the flour, baking soda, baking powder, and salt. Whisk to combine.
Combine the butter and sugar in a medium saucepan set over medium heat. Cook, stirring occasionally, until the mixture is smooth and the butter is completely melted. Transfer the mixture to the bowl of an electric mixer and beat on medium-low speed until the mixture is cool, about 4-5 minutes. Add the eggs one at a time, mixing well after each addition and scraping down the sides of the bowl as needed. Blend in the vanilla and then the cocoa mixture until smooth. With the mixer on low speed, add the flour mixture in three additions alternating with the sour cream, beginning and ending with the dry ingredients and mixing each addition just until incorporated.
Divide the batter evenly between the prepared liners, filling 2/3 of the way. Bake until a toothpick inserted in the center comes out clean, about 18-20 minutes. Let cool in the pan about 5-10 minutes, then transfer to a wire rack
To fill the cupcakes, transfer the caramel sauce to a pastry bag fitted with a small tip and inject a small amount of filling (about 1 heaping tablespoon) directly into the center of each cupcake. (I find it easiest to use the injection method when the cupcakes are still slightly warm.)
To make the caramel buttercream, place ½ cup plus 2 tablespoons of the sugar in a medium saucepan, stir in the water. Bring the mixture to a boil over medium heat. Stop stirring and let the caramel cook, gently swirling from time to time, until it is a deep amber color (test a drop on a white plate or bowl if necessary), watching it carefully to avoid burning. Remove the mixture from the heat and slowly whisk in the cream and then the salt. Set aside and let cool.
Combine the egg whites and the remaining ¼ cup plus 2 tablespoons sugar in a heatproof bowl set over a pot of simmering water. Heat, whisking frequently, until the mixture reaches 160° F and the sugar has dissolved. Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes. (The bowl should be cool to the touch.)
Reduce the speed to medium and add the butter, 2 tablespoons at a time, adding more once each addition has been incorporated. If the frosting looks soupy or curdled, continue to beat on medium-high speed until thick and smooth again, about 3-5 minutes more (or longer – don’t worry, it will come together!) Blend in the cooled caramel until smooth and completely incorporated, scraping down the sides of the bowl as needed.
Transfer the frosting to a pastry bag fitted with a large decorative tip. Pipe a swirl of frosting on each cupcake. When you are ready to serve, top each cupcake with a salted caramel if desired, right before serving.