For the cake:
2¼ cups all-purpose flour
2¼ cups sugar
1 cup plus 2 tablespoons dark unsweetened cocoa powder
2¼ tsp. baking soda
1½ tsp. baking powder
1½ tsp. salt
6 tbsp. vegetable oil
1 cup plus 2 tbsp. buttermilk
1 cup plus 2 tbsp. brewed coffee*
3 large eggs, at room temperature
1 tbsp. vanilla extract
For the filling:
1 cup sugar, divided
¼ cup water
¼ cup heavy cream
Generous pinch of sea salt, such as fleur de sel
4 large egg whites
1½ cup (3 sticks) unsalted butter, at room temperature
For the frosting:
12 oz. good quality semisweet or bittersweet chocolate, finely chopped
¼ cup unsweetened Dutch-process cocoa powder
¼ cup very hot water
1 cup plus 2 tbsp. (18 tbsp.) unsalted butter, at room temperature
1/3 cup plus 1 tbsp. confectioners’ sugar
Pinch of salt
Fleur de sel, for finishing
*Note – Coffee is used as a flavor enhancer in many chocolate cake recipes. Even if you are not a fan of coffee, don’t be afraid of this – it’s still chocolate cake, and you don’t taste the coffee.
To make the cake, preheat the oven to 350˚ F. Grease and flour the edges of 3 8-inch baking pans, shaking out the excess. Line the bottoms with rounds of parchment paper. In the bowl of an electric mixer, combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Mix on low speed to blend. Add the vegetable oil, buttermilk, coffee, eggs, and vanilla to the bowl and mix on low speed until well blended and completely incorporated. Divide the batter evenly between the prepared pans. Bake 20-25 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans about 15 minutes, then invert onto a wire rack and allow to cool completely. Remove the parchment paper.
To make the caramel buttercream filling, place ½ cup plus 2 tablespoons of the sugar in a medium saucepan. Mix 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.
To make the frosting, place the chopped chocolate in a heatproof bowl set over a pan of simmering water. Heat, stirring occasionally, until the chocolate is completely melted and smooth. Set aside and let cool to room temperature. In a small bowl, combine the cocoa powder and water and stir until smooth. In the bowl of an electric mixer, combine the butter, confectioners’ sugar and salt. Beat on medium-high speed until light and fluffy, about 5 minutes. With the mixer on low speed, gradually blend in the melted and cooled chocolate until well incorporated. Blend in the cocoa powder-water mixture until smooth.
To assemble the cake, level the cake layers if necessary. Place one of the cake layers on a cake board or serving platter. Top with half of the caramel buttercream and smooth in a thick, even layer. Place a second cake layer on top and smooth the remaining caramel buttercream over that. Place the final cake layer on top. Cover the top and sides of the cake with the chocolate frosting and smooth with an offset spatula. If desired, use additional frosting to pipe decorative accents on the cake. Refrigerate until ready to serve. Before serving, sprinkle with fleur de sel.