1Preheat the oven to 375°F. With a pastry brush, butter the bottom and sides of an 8-inch springform cake pan.
2With a whisk or electric beater, beat the egg yolks for about 1 minute, then slowly pour in the sugar. Continue beating until the mixture falls back on itself in a ribbon when the beater is lifted out of the bowl.
3Beat in the grated lemon rind and lemon juice, and with a rubber spatula, fold in the hazelnuts.
4In another bowl, and with a clean beater, beat the egg whites until they form stiff peaks on the beater when it is lifted out of the bowl. Gently but thoroughly fold the whites into the egg-yolk mixture until no streaks of white show.
5Pour the batter into the buttered pan and spread evenly to the sides with the spatula.
6Bake in the center of the oven for about 40 minutes, or until it has puffed and has begun to come slightly away from the sides of the pan. Turn off heat and let cake rest in the oven for 15 minutes. Remove the sides of the pan and turn cake out on a cake rack. Cool to room temperature.
7To make the topping, beat the cream with a whisk or electric beater until it forms soft peaks. Then gradually beat in the rum and sugar and continue to beat until stiff. With a spatula, spread over the top of the cake and serve at once.