For Sponge cake: mix egg yolks with sugar until smooth. Whip egg whites separately until very firm and add them to the mix. Slowly fold in the flour, then add grated citrus peel or vanilla flavour. Bake sponge cake for 40 minutes at moderate heat.
To prepare Cassata: mix ricotta with icing sugar until it turns into a smooth cream. Add a drop of any white sweet liqueur (optional). Add chocolate chips and candied citrus peel. Line a small tray or bowl with slices of sponge cake, put ricotta cream in the middle and top with a layer of sponge cake to seal. Refrigerate for at least an hour. Serve with seasonal fruit and decorate with icing sugar. Dolcissimo!