Preheat the oven at 180C/350F/Gas 4.
Grease and flour a round shallow 25cm/10in non-stick cake tin.
In a large bowl beat the butter and half of the sugar. Add the egg yolks and continue to beat until light and fluffy. Fold in the sieved flour.
In a separate bowl beat the ricotta lightly with a fork, add the ground hazelnuts and lemon peel. Add this to the egg and flour mixture.
In another bowl whisk the egg whites until stiff. Fold in the remainder of the sugar. Fold in the ricotta mixture very carefully.
Pour the mixture into the greased and floured cake tin and bake for 30-40 minutes. Allow to cool, remove from the tin and place on a plate.
To make the glaze, place the jam with the water in a small pan over a gentle heat to dilute until you get a smooth, fairly runny consistency.
Brush over the top of the cake and sprinkle with the grated chocolate.
Serve with sweetened mascarpone cream if desired.