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To begin the process, start with the first part of the mousse, as this needs to chill for a few hours.
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In a small saucepan on medium low heat, heat 1 cup of heavy cream until it hot, but not boiling, about 160 degrees.
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Place chocolate chunks in a large bowl and pour hot cream over the chunks, and stir until smooth. Once smooth, stir in the champagne.
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Cover mixture and place in the fridge to cool.
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To make the cake, preheat your oven to 325 degrees. Spray three 7 inch round pans or two 9 inch round pans with nonstick cooking spray and line the bottoms with a parchment paper round.
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Melt butter, then set aside.
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Separate eggs. In the clean bowl of your mixer using your paddle attachment, beat egg yolks, ½ cup of sugar, and salt on medium high speed, until the have doubled in volume and are pale, about 5 minutes.
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Place egg yolk mixture into a large bowl and throughly clean and dry your mixing bowl.
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Stir vanilla or almond extract into egg yolk mixture.
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Place egg whites in clean mixing bowl with remaining ½ cup of sugar. Beat egg whites on high until soft peaks form (when you remove the whisk attachment, your peak should curl over slightly). Do not beat to stiff peaks.
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Measure out flour.
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Scoop about a quarter of the whites over the yolk mixture and fold together gently with a few broad strokes of a rubber spatula.
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Sift about a third of the cake flour over the mixture and scoop about a third of the remaining whites over the flour, folding together gently.
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Repeat process two more times, until all egg whites and flour have been combined.
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Fold cooled melted (should be room temperature to the touch) butter into the cake batter, folding together gently.
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Pour batter into pans and bake until sides are no longer touching the pan and tops have browned, about 35-40 minutes.
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Remove from oven and cool completely.
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Once cakes have cooled and chocolate custard (1st step) is cold, continue making the mousse.
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In a clean bowl, beat remaining 1 and ½ cup heavy whipping cream to stiff.
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Fold whipped cream into chocolate mixture.
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To assemble the cake, place 1st layer on the a cardboard cake round (preferably the same size as the cake). Brush with champagne.
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Spread a thick layer of mousse onto the 1st layer and stack with next layer. If making a 3-tiered cake, repeat process.
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Refrigerate cake while you make the ganache.
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To make the ganache, heat heavy whipping cream in a small sauce pan on medium low heat, until hot but not boiling.
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Place chocolate in a bowl and pour hot cream over chocolate. Stir until smooth.
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Place cake on a wire cooling rack with foil underneath the rack to catch excess ganache.
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Pour ganache over cake until it is completely covered.
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Allow cake to sit and cool, about 3 hours.
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Enjoy the within 2 days.