White and Dark Chocolate Mousse Cake with Caramelized White Chocolate

Adapted from 2 awesome recipes, and made for a school chocolate bakeoff (no I did not win, but it was still delicious, despite some mishaps with the caramelized white chocolate and a melting white mouse). I used a Payard recipe for a milk and dark chocolate mousse cake, including chocolate cake layers between the mousse, but substituted in white chocolate for the milk. I intended to caramelize the white chocolate following David Lebovitiz recipe, but the chocolate never left the crumbly, chalky phase, and thus I left it in the oven too long in the hopes that it might, leading it to take on more of a burnt caramel flavor rather than the sweet caramel I hoped for. Fearing that the burned flavor would be overwhelming in a mousse, I opted to substitute in regular white chocolate for the mousse, and use the caramelized white chocolate as a thin layer over the chocolate cake between mousse layers. It turned out well, though the caramelized white chocolate was mostly lost. I also didn't have a pastry ring in which to assembly the cake, and the springform pan worked fine for the bottom mousse layer, but the top one ended up melting over the edges, even after half an hour in the freezer, so next time I'd definitely do it in the suggested ring.

White and Dark Chocolate Mousse Cake with Caramelized White Chocolate

Chocolate Génoise:
3/4 plus 1 tablespoon all-purpose flour, sifted
3/4 cup unsweetened cocoa powder, sifted
6 large eggs
1 cup sugar
2 tablespoons unsalted butter, melted

White and Dark Chocolate Mousses:
3 1/2 cups heavy cream
5 ounces white chocolate, finely chopped
5 ounces bittersweet chocolate, finely chopped

Sugar Syrup:

1/4 cup (50 grams) sugar

Caramelized White Chocolate Layer:

12 oz. white chocolate, chopped into small pieces (higher cocoa butter percentage works better)

1/2 cup heavy cream

Make the chocolate génoise:
1. Preheat the oven to 350°F. Butter a 9-inch round cake pan. Dust the pan with cocoa powder and tap out the excess.

2. Whisk together the flour and cocoa powder in a medium bowl; set aside.

3. Fill a pot one-third full with water and bring to a simmer. Combine the eggs and sugar in the bowl of a standing mixer. Place the bowl over the simmering water and whisk until the mixture is warm to the touch. Transfer the bowl to the mixer stand and, using the whisk attachment, beat on medium speed until the mixture triples in volume and forms a thick ribbon when the whisk is lifted, about 8 minutes. Using a large rubber spatula, gently fold in the flour mixture. Fold in the melted butter. Scrape the batter into the pan and smooth the top.

4. Bake the cake for 20 to 25 minutes, until it has pulled away from the side of the pan and the top springs back when lightly touched. Cool the cake in the pan for 10 minutes. Unmold the cake on a rack and cool completely, right side up.

Make the caramelized white chocolate:

5. Preheat the oven to 250F.

6. Distribute the white chocolate on a rimmed baking sheet and heat for ten minutes.

7. Remove it from the oven and spread it with a clean, dry spatula.

8. Continue to cook for an additional 40-60 minutes, stirring at 10 minute intervals. At some points it may look lumpy and chalky, but keep stirring and it will smooth out and caramelize (or not, as in my case).

9. Cook until the white chocolate is deep-golden brown, and caramelized (or until it tastes caramelized, even if still lumpy).

10. If it's lumpy, scrape it into a bowl and smooth it out with an immersion blender, or in a food processor, and add up to 1/2 cup heavy cream to smooth it out and make it pourable.

Make the white and dark chocolate mousses:
11. In the bowl of an electric mixer, using the whisk attachment, beat 2 1/2 cups of the heavy cream to soft peaks. Scrape half of the whipped cream into a medium bowl. Cover both bowls and refrigerate.

12. Put the white and dark chocolate in two separate medium bowls. Bring the remaining 1 cup cream to a boil in a small saucepan. Pour 1/2 cup of the hot cream over the white chocolate and the remaining 1/2 cup over the bittersweet chocolate. Gently whisk the white chocolate until it is completely melted and the mixture is smooth. Whisk the dark chocolate until smooth.

13. Let the chocolate mixtures cool for 10 minutes. Fold the white chocolate mixture into one bowl of whipped cream. Fold the dark chocolate into the remaining whipped cream.

Make the sugar syrup:
14. Combine 1/4 cup (59 grams) water and the sugar in a small saucepan. Bring to a boil over medium-high heat, stirring to dissolve the sugar. Let the syrup cool to room temperature.

Assemble the cake:

15. Using a serrated knife, slice the cake horizontally in half. Cup a strip of parchment paper to line the inside of a 9x4-inch cake ring. Line the mold with the strip and place it on a serving plate. Place a 9-inch cardboard cake round in the bottom of the cake ring. Place the bottom cake round, cut side up, in the ring. Brush the cake generously with the sugar syrup. Spread 1/2 of the caramelized white chocolate over the cake. Scrape the dark chocolate mousse onto the cake and smooth it into an even layer. Top with the second cake round, cut side down. Brush the cake generously with syrup. Spread the rest of the caramelized white chocolate. Scrape the white chocolate mousse onto it, smoothing it into an even layer. Chill the cake for at least 2 hours and up to 24 hours.

16. To unmold the cake, lift it off the plate, holding it with your palm against the cardboard round. Using a hairdryer or warm damp towel, briefly warm the cake ring mold. Lift off the ring mold and peel off the parchment strip. Allow the cake to stand at room temperature for 30 minutes before serving.