pastry:
sift the flour and the cocoa into a bowl and mix with the sugar. dice the butter and rub into the dry ingredients until the mixture resembles fine breadcrumbs. add the sherry and make a smooth dough using your fingers.
take 3/4 of the dough, roll it out and line a loose-based fluted flan tin (23cm/9in). put in the fridge for 20 minutes.
preheat the oven to 200˚C.
filling:
place the egg yolks and sugar in a bowl and beat with an electric whisk until light and fluffy. add the ricotta and combine well using a wooden spoon. stir in the lemon rind, mixed peel and chocolate. spoon into the pastry shell.
roll out the remaining 1/4 of the dough and cut it into 1.5cm strips. arrange them on top of the filling into a lattice pattern.
bake at 200˚C for 15 minutes, then turn the oven to 180˚C and bake for 30 -35 minutes until golden brown on top.
leave to cool in the tin.