For the sponge cake: 2/3 cup flour 2/3 cup potato starch 3/4 cup sugar 6 eggs Butter and flour for cake pan as needed For the Chantilly cream: 2 cups milk 2 eggs 2 egg yolks 1/2 cup sugar 1/4 cup flour Zest of 1 lemon 1/2 vanilla bean 1/2 cup heavy cream 1/2 cup confectioner's sugar For the syrup: 1 cup water 1/2 cup Kirsch liqueur 2/3 cup sugar
To make sponge cake, beat eggs and sugar in a mixer until creamy, firm and doubled in volume. Sift together flour and potato starch and carefully fold in with a spatula, a little at a time. Pour into two 7-inch buttered and floured round cake pans. Bake in a preheated 320°F oven for 45 minutes. Remove from the oven and set aside to cool.
Make the Chantilly cream, which is a mixture of whipped cream and pastry cream. Whip very cold heavy cream with confectioner's sugar and refrigerate until ready to use. Prepare pastry cream: Slice vanilla bean open lengthwise, scrape out seeds and collect in a bowl. With a peeler, remove zest from lemon, carefully avoiding the pith, as it is quite bitter. In a pot, bring milk, lemon zest and vanilla seeds to a boil. As soon as it starts boiling, turn off the heat. In a bowl, beat whole eggs and egg yolks with sugar. Stir in sifted flour and mix until smooth. Thin with a very small amount of milk, stirring constantly. Pour in the remaining milk through a sieve to remove lemon zest. Return this mixture to the pot and heat over low heat for 5 - 10 minutes, stirring constantly (it should boil for at least 2 - 3 minutes). Remove cream from the heat and set aside to cool completely. Fold pastry cream into reserved whipped cream. Prepare syrup to soak the sponge cake with: Bring water and sugar to a boil in a saucepan. As soon as the sugar has dissolved, remove from the heat and stir in Kirsch liqueur.
Assembling the cake: With a bread knife, cut one of the two sponge cakes into 1/2-inch horizontal slices.
Line an 8-inch glass dish with parchment paper. Place one slice of sponge cake into the dish.
Brush sponge cake with Kirsch syrup.
Spoon one layer of Chantilly cream and cover with another slice of sponge cake. Continue this process until there are 3 - 4 layers of sponge cake. Refrigerate for about 30 minutes.
Slice the other sponge cake into 1/4 - 1/2-inch cubes.
Turn out the cake onto a platter.
Cover the outside with the remaining cream.
Sprinkle sponge cake cubes over the top, covering the whole cake.