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The day before, preheat the oven to 160C/Gas 3/fan oven 140C. Brush a 25cm/10in springform cake tin with the melted butter and then dust with flour.
Make the sponge. Using an electric mixer fitted with balloon whisks, beat the eggs with the sugar until pale and airy. The mixture should be thick enough to leave a ribbon trail on the surface when the whisks are lifted – it takes about 8-10 minutes to get to this stage. Gently fold in the flour and lemon zest using the whisks or a spoon, trying not to deflate the beaten eggs and sugar.
Working quickly, transfer to the prepared cake tin and bake for about 30 minutes until firm to the touch (the cake should not have risen). Stand the tin on a wire rack and leave to cool, then remove the cake from the tin.
Make the ricotta cream. Finely chop the crystallised fruits and frozen chocolate. Mix the ricotta with the sugar and gently incorporate the fruit and chocolate. Place in the fridge until you are ready to assemble the cake.
Make the icing just before assembling the cake. Mix the icing sugar with the lemon juice and 1 tablespoon milk, then add a drop of green food colouring to give the icing a subtle green tinge, and beat vigorously. You should have a spreadable paste with a thick, non-liquid consistency – you may need a little extra milk.
Assemble the cake. Melt the jam with 1 tablespoon liqueur and 1 tablespoon water. Slice the sponge in half horizontally. Place the less perfect half on a flat serving plate and spread with half the jam mixture. Spread all the ricotta mixture on top. Spread the remaining jam mixture on the remaining sponge, then turn it over and put it jam side down on the ricotta cream. Spread the icing evenly across the top and smooth over with a palette knife, then place in the fridge for at least 1 hour (or for up to two days).
On the day, a few hours before serving decorate the top of the cake with crystallised fruits, peel and angelica as the fancy takes you.