1. Brush butter and flour mixture inside two 10-inch cake pans. Reserve.
2. Combine the eggs, sugar, vanilla, and salt in a mixer bowl. Heat over simmering water until the mixture reaches about 110 F (43 C), whipping continuously. Remove from the heat and whip at high speed until the mixture has cooled and has a light and fluffy consistency (the batter has to be stiff enough for a drop to remain on top of the batter if dropped from a spoon and reluctantly settle).
3. Sift the flour. Mix the hazelnuts and chocolate, then carefully fold into the batter by hand. Fold in the melted butter. Divide the batter between the prepared pans.
4. Bake immediately at 400 F (205 C) until the cake springs back when pressed lightly on top, approximately 15 minutes. Allow the sponges to cool completely before removing them from the pans.
Note: Add about one-quarter of the sugar to the nuts if you are using a food processor to grind them. The sugar will absorb the oil released by the nuts due to heat produced by the friction.