Preheat the oven to 180°C.
Grease and flour a 25cm round shallow non-stick cake tin.
Beat the butter and half the sugar in a bowl until light, then add egg yolks and fold in flour.
Lightly beat the ricotta and mix with ground hazelnuts and lemon peel before adding to the flour mixture.
Whisk egg whites until stiff, fold in the remaining sugar, and carefully fold into the ricotta mixture.
Pour mixture into the tin and bake for 30-40 minutes.
Heat the jam and water in a pan over low heat until smooth and runny.
Brush the glaze over the cooled cake and sprinkle with grated chocolate.
Serve the cake with sweetened mascarpone cream if desired.