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Triple-Chocolate Mousse Cake

August 1, 2011

One of my lunch/study buddies from grad school bought a house with a nice in-ground pool a couple months ago, and now that he and his family are settled in, he invited me and our other school friends over for a barbeque and pool party this past weekend. Our other friend recently moved back into our area after finishing a residency program out of state as well, and as he (and his fianceé) love chocolate desserts, I had promised him earlier in the spring that I would make him this triple-chocolate mousse cake (from the Cook’s Illustrated Nov/Dec 2009 issue) when he moved back. What better opportunity for this fancy dessert than to celebrate his return and our reunion all in one?

This cake is three distinct layers: the bottom is a dense flourless chocolate cake, the middle is a chocolate mousse, and the top is a light white chocolate mousse. One issue which affected the aesthetics (but not the taste) was that although the directions do say that the bottom layer collapses as it cools, in my case it also pulled away from the sides of the pan, allowing the middle layer to cover up the outside edge of the bottom layer. So from the outside, the cake appears to have a double-thick chocolate mousse layer under the white chocolate. I’m not sure how I would prevent this from happening if I make it again in the future and need it to be more “picture perfect”; I debated moving the whole cake from the 9.5-inch to a 9-inch springform pan, but being a flourless cake, I think it would have broken in half if I had tried to transfer it from one pan bottom to the other. Perhaps I will have to email the people at Cook’s Illustrated and see if they have ever run into the same problem with this recipe. Either way it was delicious, and I took home a completely clean pan when the party was over!

One note: Although we cut this cake with a regular knife, the recommended method for the best presentation is to use a cheese wire to achieve very clean cuts with no smearing of the layers.

Triple-choclate mousse cake

Serves 12 to 16

Ingredients

Bottom layer

  • 6  tablespoons (3/4 stick) unsalted butter, cut into 6 pieces
  • 7  ounces bittersweet chocolate, chopped fine
  • 3/4  teaspoon instant espresso powder
  • 1 1/2  teaspoons vanilla extract
  • 4  large eggs, separated
  • Pinch salt
  • 1/3  cup packed (about 2 1/2 ounces) light brown sugar

Middle layer

  • 2  tablespoons cocoa powder, preferably Dutch-processed
  • 5  tablespoons hot water
  • 7  ounces bittersweet chocolate, chopped fine
  • 1 1/2  cups cold heavy cream
  • 1  tablespoon granulated sugar
  • 1/8  teaspoon salt

Top layer

  • 3/4  teaspoon powdered gelatin
  • 1  tablespoon water
  • 6  ounces white chocolate chips
  • 1 1/2  cups cold heavy cream
  • Shaved chocolate or cocoa powder for serving

Directions

  1. For the bottom layer: Adjust oven rack to middle position and heat oven to 325 degrees. Grease bottom and sides of 9 1/2-inch springform pan. Melt butter, chocolate, and espresso powder in large heatproof bowl set over saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and cool mixture slightly, about 5 minutes. Whisk in vanilla and egg yolks; set aside.
  2. In stand mixer fitted with whisk attachment, beat egg whites and salt at medium speed until frothy, about 30 seconds. Add half of brown sugar and beat until combined, about 15 seconds. Add remaining brown sugar and beat at high speed until soft peaks form when whisk is lifted, about 1 minute longer, scraping down sides halfway through. Using whisk, fold one-third of beaten egg whites into chocolate mixture to lighten. Using rubber spatula, fold in remaining egg whites until no white streaks remain. Carefully transfer batter to prepared springform pan, gently smoothing top with offset spatula.
  3. Bake until cake has risen, is firm around edges, and center has just set but is still soft (center of cake will spring back after pressing gently with finger), 13 to 18 minutes. Transfer cake to wire rack to cool completely, about 1 hour. (Cake will collapse as it cools.) Do not remove cake from pan.
  4. For the middle layer: Combine cocoa powder and hot water in small bowl; set aside. Melt chocolate in large heatproof bowl set over saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and cool slightly, 2 to 5 minutes.
  5. In clean bowl of stand mixer fitted with whisk attachment, whip cream, granulated sugar, and salt at medium speed until mixture begins to thicken, about 30 seconds. Increase speed to high and whip until soft peaks form when whisk is lifted, 15 to 60 seconds.
  6. Whisk cocoa powder mixture into melted chocolate until smooth. Using whisk, fold one-third of whipped cream into chocolate mixture to lighten. Using rubber spatula, fold in remaining whipped cream until no white streaks remain. Spoon mousse into springform pan over cooled cake and gently tap pan on counter 3 times to remove any large air bubbles; gently smooth top with offset spatula. Wipe inside edge of pan with damp cloth to remove any drips. Refrigerate cake at least 15 minutes while preparing top layer.
  7. For the top layer: In small bowl, sprinkle gelatin over water; let stand at least 5 minutes. Place white chocolate in medium bowl. Bring ½ cup cream to simmer in small saucepan over medium-high heat. Remove from heat; add gelatin mixture and stir until fully dissolved. Pour cream mixture over white chocolate and whisk until chocolate is melted and mixture is smooth, about 30 seconds. Cool to room temperature, stirring occasionally, 5 to 8 minutes (mixture will thicken slightly).
  8. In clean bowl of stand mixer fitted with whisk attachment, whip remaining cup cream at medium speed until it begins to thicken, about 30 seconds. Increase speed to high and whip until soft peaks form when whisk is lifted, 15 to 60 seconds. Using whisk, fold one-third of whipped cream into white chocolate mixture to lighten. Using rubber spatula, fold remaining whipped cream into white chocolate mixture until no white streaks remain. Spoon white chocolate mousse into pan over middle layer. Smooth top with offset spatula. Return cake to refrigerator and chill until set, at least 2½ hours.
  9. To serve: Garnish top of cake with chocolate curls or dust with cocoa. Run thin knife between cake and side of springform pan; remove side of pan. Run cleaned knife along outside of cake to smooth sides. Cut into slices and serve.

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