For this type of chocolate mousse cake, baked cake layers are filled with chocolate mousse. The recipe is adapted from one by my friend Loretta Sartori, who runs a baking education program in Melbourne, Australia. I love the fact that the mousse sets only from the addition of the chocolate, without the use of gelatin, making it so much more delicate. Sometimes Loretta adds some prunes that have been soaked in Armagnac to the bottom layer of mousse filling. Make sure to prepare this the day before you intend to serve it to give the mousse filling plenty of time to chill and set.
Make one 9-inch cake, about 12 servings
One Cocoa Génoise, below, baked, cooled, and cut into 3 layers (only 2 layers will be used to assemble the cake; wrap and freeze the extra layer for another use)
1/3 cup water
1/4 cup sugar
2 teaspoons vanilla extract
Milk Chocolate Mousse
3 cups heavy whipping cream
1/3 cup sugar
6 large egg yolks
12 ounces best-quality milk chocolate, cut into 1/4-inch pieces
4 ounces bittersweet (not unsweetened) chocolate, cut into 1/4-inch pieces
1 cup heavy whipping cream
2 cups milk chocolate shavings
One 9-inch springform pan
For the syrup, bring the water and sugar to a boil in a small saucepan. Pour the syrup into a small bowl, cool it, and stir in the vanilla.
For the mousse, combine 1 cup of the cream and the sugar in a medium saucepan and whisk to mix. Place over low heat and bring to a full rolling boil. Meanwhile set a fine strainer over a clean glass or stainless steel bowl and place them near the burner where you are heating the liquids. Whisk the egg yolks in a small bowl to break them up. When the liquid boils, whisk about a third of it into the yolks. Return the liquid to a boil, and beginning to whisk before pouring, pour the yolk mixture into the boiling liquid. Whisk constantly until the cream thickens slightly – it won’t be very thick – about 10 or 15 seconds after adding the yolks. Remove the pan from the heat, never ceasing to whisk. Quickly strain the sauce into the prepared bowl. Remove the strainer and set it over the saucepan. Whisk the sauce continuously for about 30 seconds to cool it down so that it doesn’t scramble.
Combine the milk and bittersweet chocolates in a large bowl and pour the hot custard cream over them. Shake the bowl to make sure all the chocolate is submerged, then let it stand for 2 minutes. Whisk smooth and cool to room temperature.
Whip the cream until it holds a soft peak and set it aside refrigerated.
After the chocolate cream has cooled, place one of the cake layers in the bottom of the prepared pan. Use a brush to sprinkle the layer with half the syrup.
Remove the whipped cream from the refrigerator and re-whip by hand if it has become liquid on the bottom. Quickly fold the cream into the cooled chocolate custard. Pour half the mousse on the layer in the pan and use a small offset spatula to spread it smooth.
Top with the other cake layer without pressing it into the mousse, and sprinkle it with the remaining syrup. Top with the remaining mousse, and spread the top of the cake smooth.
Refrigerate the cake overnight, covered, to set the mousse.
To unmold the cake, run a small knife between the dessert and the inside of the pan. Unbuckle the side of the pan and lift it off.
Whip the cream and spread it smoothly all over the outside of the cake. Use a metal spatula to press the chocolate shavings against the side of the cake. Sprinkle the remaining shavings all over the top of the cake and use the spatula to sweep them into an even layer.
If your sprig form pan has a flat base, use a large spatula to loosen the cake from the pan base to a platter.
Serving: Cut wedges of the cake at the table, using a long, thin-bladed knife. Wipe the knife with a wet cloth every time you cut through the cake to avoid tracking crumbs into it.
Storage: Keep the cake refrigerated until you intend to serve it.
Makes one tall 9-inch layer
3 large eggs
3 large egg yolks
2 teaspoons vanilla extract
1/2 cup sugar
1/3 cup all-purpose flour
1/3 cup cornstarch
1/4 cup alkalized (Dutch process) cocoa powder
One 9-inch springform pan, 2 1/2 to 3 inches deep, buttered and the bottom lined with a disk of parchment or buttered wax paper
Set a rack in the middle level of the oven and preheat to 350˚F.
Half-fill a saucepan with water and bring it to a boil over medium heat. Decrease the heat so that the water still boils gently.
Combine the eggs, yolks, vanilla, and salt in the bowl of an electric mixer. Whisk by hand to break up the eggs, then whisk in the sugar in a stream. Place the bowl over the simmering water and gently whisk until the mixture is lukewarm, about 115˚F.
Place the bowl on the mixer with the whisk and whip on medium to high speed until the foam is more than tripled in volume and very much lightened in color, 3 to 4 minutes. The outside of the mixer bowl will fell cool to the touch.
While the egg mixture is whipping, stir the flour, cornstarch, and cocoa powder together and place a strainer or sifter near them.
Remove the bowl from the mixer and sift a third of the flour mixture over the egg foam. Use a large rubber spatula to fold in the flour mixture, making sure you dig down to the bottom of the bowl every time you pass through, so that no lumps of flour accumulate there. Continue adding the next third of the flour mixture, folding until it is absorbed, ending with the last third.
Scrape the batter into the prepared pan and smooth the top.
Tilt the pan so that the batter runs all the way to the top of the pan all around the inside – this helps the layer to bake straight and flat instead of doming in the center.
Bake the génoise until it is well risen and feels firm when pressed in the center with a fingertip, 25 to 30 minutes.
Immediately unmold the cake to a rack to cool. Cover the cake with another rack and invert so that it cools right side up.
Storage: Double wrap the layer in plastic wrap, keep at room temperature, and use within 24 hours. Or freeze the layer for up to a month. Defrost the layer before assembling the finished cake.