Triple-Chocolate Mousse Cake

Serves 12 to 16

    Base Layer

  1. Adjust an oven rack to the middle position and heat the oven to 325°F.
  2. Grease the bottom and sides of a 9 1/2 by 3 inch springform pan.
  3. Melt the butter, chocolate, and espresso powder in a large heat-proof bowl set over a saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth.
  4. Remove from the heat and cool the mixture slightly, about 5 minutes.
  5. Whisk in the vanilla and egg yolks and set aside.
  6. In a clean bowl of a stand mixer fitted with a whisk attachment, beat the egg whites and salt at a medium speed until frothy, about 30 seconds.
  7. Add half of the brown sugar and beat until combined, about 15 seconds.
  8. Add the remaining brown sugar and beat at high speed until soft peaks form, about 1 minute longer, scraping down the sides halfway through.
  9. Using a rubber spatula, fold in the remaining whites until no streaks remain.
  10. Transfer the batter to the springform pan, gently smoothing the top with a spatula.

    11. Middle Layer

  11. Combine the cocoa powder and hot water in a small bowl and set aside.
  12. Melt the chocolate in a large heatproof bowl set over a saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth.
  13. Remove from the heat and cool slightly, 2 to 5 minutes.
  14. In the clean bowl of a stand mixer fitted with the whisk attachment, whip the cream, sugar, and salt at medium speed until it begins to thicken, about 30 seconds.
  15. Increase the speed to high and whip until soft peaks form, 15 to 60 seconds.
  16. Whisk the cocoa powder mixture into the melted chocolate until smooth.
  17. Using a whisk fold one-third of the whipped cream into the chocolate mixture.
  18. Using a rubber spatula, fold in the remaining whipped cream until no streaks remain.
  19. Spoon the mousse over the cooled cake and gently tap the pan on the counter 3 times to remove any large air bubbles.
  20. Gently smooth the top with an offset spatula.
  21. Wipe the inside edge of the springform pan with a damp cloth to remove any drips.
  22. Refrigerate the cake at least 15 minutes while preparing the white chocolate mousse.

    23. Top Layer

  23. In a small bowl, sprinkle the gelatin over the water and let stand at least 5 minutes.
  24. Place the white chocolate in a medium bowl. Bring 1/2 cup of the cream to a simmer in a small saucepan over medium-high heat.
  25. Remove from the heat and add the gelatin mixture and stir until fully dissolved.
  26. Pour the cream mixture over the chocolate and whisk until the chocolate is melted and the mixture is smooth, about 30 seconds.
  27. Cool to room temperature, stirring occasionally, 5 to 8 minutes, until thickened.
  28. In the clean bowl of a stand mixer fitted with the whisk attachment, whip the remaining 1 cup cream at a medium speed until it begins to thicken, about 30 seconds.
  29. Increase the speed to high and whip until soft peaks form when the whisk is lifted, 15 to 50 seconds.
  30. Using a whisk, fold one-third of the whipped cream into the white chocolate mixture to lighten.
  31. Using a rubber spatula fold the remaining whipped cream into the white chocolate mixture until no white streaks remain.
  32. Spoon the white chocolate mouse into the pan over the bittersweet chocolate mousse layer.
  33. Smooth the top with an offset spatula. Return the cake to the refrigerator and chill until set, at least 2 1/2 hours.

    34. To Serve

  34. Garnish the top with chocolate curls (if using).
  35. Run a thin knife around the inside of the springform pan and remove the side of the pan.
  36. Smooth the side of the cake with a clean knife. Cut into slices and serve.

"The Best of America's Test Kitchen 2010" Cooks Magazine. April, 2010: page 64.