Serves: 2 6-inch cakes
For the Cake: (though the original recipe yields two 9 inch cakes, I have tall 6 inch pans, so I was able to get two very tall 6 inch cakes with the same batter)
4 oz unsweetened chocolate, finely chopped
¼ cup (3/4 oz) Dutch-process cocoa powder
½ cup hot water
1¾ cups (12¼ oz) sugar
1¾ cups (8¾ oz) all-purpose flour
1½ teaspoons baking soda
1 teaspoon salt
1 cup buttermilk, at room temperature
2 teaspoons vanilla extract
4 large eggs plus 2 large egg yolks, at room temperature
12 tablespoons (1½ sticks) unsalted butter, cut into 12 pieces, at room temperature
For the caramel:
1 cup sugar
¼ cup water
¼ teaspoon fresh lemon juice
2 cups heavy cream
1 teaspoon pure vanilla extract
For the Caramel-Coffee Mousse:
2½ teaspoons instant coffee (3 single-serving packets)
1 envelope unflavored gelatin (about 2 teaspoons)
Caramel (see above)
¼ cup crème fraîche
For the ganache:
9 ounces bitterrsweet chocolate, chopped
1 cup heavy cream
For the caramel:
In a medium saucepan, combine the sugar with the water and lemon juice and bring to a boil over moderately high heat.
Using a wet pastry brush, wipe down the side of the saucepan.
Simmer the syrup undisturbed until it begins to brown around the edges, 4 to 5 minutes.
Swirl the pan, then simmer until the caramel turns a medium-dark amber, about 3 minutes longer. Remove from the heat.
Carefully add ½ cup of the cream in a steady stream. Stir with a long-handled wooden spoon to combine, then add the remaining 1½ cups of cream. Stir in the vanilla.
Transfer the caramel to a bowl and refrigerate until very cold, at least 3 hours or overnight.
For the cake:
Preheat oven to 350 F. Spray two 9-inch cake pans with nonstick cooking spray with flour (you can line the bottoms with parchment as well if you want. I used two tall 6 inch pans).
Add the unsweetened chocolate, cocoa power and water to a heatproof bowl and set over a pan of barely simmering water. Stir occasionally until the chocolate is completely melted (the mixture will be thick). Whisk in ½ cup of the sugar until well combined and the mixture becomes glossy.
Remove the bowl from the pan and set aside to cool slightly.
In a medium bowl, whisk together the flour, baking soda and salt. In a measuring cup, whisk the buttermilk and vanilla together.
In the bowl of a stand mixer fitted with the whisk attachment, combine the eggs and egg yolks. Beat on medium-low speed until combined, about 10 seconds. Add the remaining 1¼ cups of sugar and beat on high speed until the mixture is fluffy and pale in color, about 2-3 minutes.
Remove the whisk attachment and replace with the paddle. Pour the cooled chocolate mixture into the mixer bowl and beat on medium speed until well combined. Scrape down the sides of the bowl.
Add the butter, one piece at a time, mixing for about 10 seconds after each addition.
With the mixer on low, alternately add the flour mixture (in 3 additions) and the buttermilk mixture (in 2 additions), starting and ending with the flour mixture. The batter may look slightly curdled. Beat until thoroughly combined.
Divide the batter between the prepared cake pans, smoothing the top until even. Bake for 25-30 minutes (took longer for my layers to bake), or until a toothpick inserted in the center comes out with just a few moist crumbs attached. Remove the pans to a wire rack and let the cakes cool for 10 minutes. Turn the cakes out of the pans and allow to cool completely, at least 2 hours.
For the coffee-caramel mousse:
In a small microwave-safe bowl, dissolve the instant coffee in 3 tablespoons of cold water. Sprinkle on the gelatin and let stand until softened, about 5 minutes. Using an electric mixer, beat the caramel with the crème fraîche just until soft peaks form. Working quickly, microwave the coffee-gelatin mixture at 10-second intervals just until the gelatin is completely dissolved and hot to the touch. Quickly whisk the hot gelatin into the caramel.
Using a sharp pointed serrated knife, cut a cone out of the center of both cake layers so that you have a cone-shapped recess in each cake layer. Be sure to leave enough cake around the sides and at the bottom otherwise it may not be able to stand. Also make sure the cones are the same sizes so when assembled a clean diamond mousse filling will form.
Fill each recess with the mousse. Use an icing spatula to make the mousse level with the top of the cake.
Place the mousse-filled layers in the fridge and let chill until firm, about 1 hour.
Remove the layers from the fridge. Carefully invert one layer onto the other, making sure the edges of the cake, and the mousse line up. Remove the cake round from the top of the cake.
Make the ganache:
Place the chocolate into a medium bowl. Heat the cream in a small sauce pan over medium heat. Bring just to a boil, watching very carefully because if it boils for a few seconds, it will boil out of the pot. When the cream has come to a boil, pour over the chopped chocolate, and whisk until smooth.
Allow the ganache to cool slightly before pouring over the cake. Start at the center of the cake and work outward. For a fluffy frosting or chocolate filling, allow it to cool until thick, then whip with a whisk until light and fluffy.
I made the caramel and cakes a day before, and just before assembling I made the mousse and scooped out the cakes.
I dusted the top with some cocoa powder and put some coffee beans for decoration. Store in the refrigerator.