Volcano Surprise with Lemon Mousseline Cake

Put the egg yolks, sugar, grated lemon rind, and vanilla in a bowl, and mix well with a whisk until smooth and thick. Add the potato starch and flour, and mix with a whisk.
Preheat the oven to 350 degrees. In another bowl, beat the egg whites until firm. Add about one-third of the egg whites to the egg-yolk mixture, and mix with a whisk to lighten the batter. Fold the remainder of the egg whites into the batter. Pour the batter into a buttered 9-inch springform pan.
Bake on a cookie sheet in the 350-degree oven for approximately 35 minutes. The cake should be nice and puffy as it comes out of the oven. Let cool in the pan in a warm place. The cake will shrink slightly but remain moist and spongy inside.
Butter the center of a round ovenproof platter and slice the cake horizontally into four layers. Place one of the cake slices on the buttered platter. Spoon the ice cream onto the cake slice, mounding it in the center, and smooth with a spatula.
Cover the top of the ice-cream mound completely with cake, breaking the remaining slices of cake into large pieces, as needed, to make them fit snugly and over the filling entirely. (Any leftover cake can be frozen for future use.) Sprinkle the cognac over the cake. At this point, the dessert can be placed in the freezer while you prepare the rest of it.
In the bowl of a mixer fitted with a whisk attachment, beat the egg whites until firm. Meanwhile, mix the egg yolks and the vanilla in a separate bowl with a fork just until blended. When the egg whites are firm, add the sugar fairly quickly, and continue beating with the whisk for another 15 to 20 seconds to incorporate it. Gently fold the egg yolks into th egg whites with a spatula. Spoon some of the souffé mixture on top of the ice-cream cake, and smooth and round it into a conical shape.
Preheat the oven to 400 degrees. You should have approximately a 1-inch thickness of the soufflé mixture on the cake, with the soufflé higher in the center. Spoon the rest of the soufflé mixture into a pastry bag fitted with a fluted tip, and make a spiral decoration all around the bottom of the volcano. Continue decorating the cake as you like with the remainder of the soufflé mixture.
Embed the hollowed-out half-lemon shell in the center of the top of the volcano. Work quickly, so the ice cream and the soufflé coating remain firm. Bake in the 400-degree oven for 12 to 15 minutes. The outside of the cake should be nicely browned, and it should be just set inside.