Preheat oven to 350F (180C).

Grease, flour, and line a 15 1/2- x 15 1/2-inch jellyroll pan with waxed paper.

Beat egg yolks with 3/4 cup sugar until the mixture falls in a ribbon when beaters are lifted.

Stir in 1/4 cup all-purpose flour and 2 tablespoons of vanilla extract.

Beat egg whites until stiff but not dry.

Gently fold the egg whites into the yolk mixture.

Pour the batter into the prepared pan, spreading it evenly.

Bake for 20 minutes.

Let the genoise cool completely.

To make the mousse, combine 1 cup sugar, 1 cup lemon juice, and 1 cup water in a saucepan.

Add the shredded peels of 3 oranges, 3 grapefruits, and 3 lemons to the saucepan.

Cook until the citrus peels are tender.

Drain the peels, reserving the syrup for another use.

Using half of the 1 pound of white chocolate, make chocolate curls with a vegetable peeler.

Place the chocolate curls on a waxed paper-lined tray and freeze to harden.

Melt the remaining white chocolate in the top of a double boiler over barely simmering water.

Cool the melted chocolate.

Whip 3 cups of heavy whipping cream until stiff.

Alternately fold half of the melted chocolate, half of the candied citrus peel, and half of the 1/2 cup of chopped pine nuts into the whipped cream.

Add the remaining melted chocolate, half of the remaining candied peel, and half of the remaining pine nuts until incorporated into the cream.

Cut the cooled genoise crosswise into 4 equal rectangular layers.

Spread some of the white chocolate mousse on the first layer.

Top with another cake layer.

Spread with more mousse.

Repeat with the next 2 layers, spreading mousse over the top and sides of the cake.

Cover the top and sides of the cake with the frozen chocolate curls.

Delicately place the reserved candied citrus peel and pine nuts over the chocolate curls.

Sprinkle with powdered sugar.