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Special equipment: 2 pastry bags fitted with round tips
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For the buttercream: Combine the sugar and egg whites in a double boiler and heat to 120 degrees F.
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Transfer the egg white mixture to a stand mixer fitted with a whisk attachment.
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Add the lemon juice and salt and beat on high speed until completely cooled, about 15 minutes.
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Add the butter piece by piece and whip until light fluffy, about 10 minutes.
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Fill a pastry bag fitted with a round tip and reserve the rest.
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For the vanilla bean white chocolate mousse: Whip the egg yolks on high speed in the bowl of a stand mixer fitted with a whisk attachment until light and foamy, 3 to 4 minutes.
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Heat the sugar, vanilla bean paste and water to 120 degrees C in a small pot over medium-high heat.
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Add the sugar syrup to the whipping yolks and beat until cool, about 15 minutes.
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Once cooled, fold in the whipped cream.
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Melt the white chocolate over a double boiler until 110 degrees F., about 10 minutes.
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Add the melted chocolate to the egg and cream mixture and fold to combine.
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Fill a pastry bag fitted with a round tip and reserve the rest.
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For the vanilla syrup: Combine the vanilla bean paste, sugar and water in a small saucepan over medium-high heat until the sugar is dissolved.
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Cool completely.
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To assemble the cake: Split the cake rounds in half to make 4 layers.
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Brush the top of each with 3 tablespoons of the vanilla syrup.
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Place 1 round on desired platter and pipe the buttercream around the top edge of the cake to create a dam.
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Fill the dam with a layer of the white chocolate mousse.
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Top with a second cake round and repeat piping and filling with the remaining layers, leaving the top undecorated.
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Place the stack in the freezer until firm, 30 minutes to 1 hour.
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Remove from the freezer and lightly frost the whole cake with some of the buttercream.
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Place the cake back in the freezer to firm up, another 30 minutes to 1 hour.
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Frost and decorate the cake as desired with the remaining buttercream and mousse.