Mash the ricotta and beat well with the egg yolks, stir in the flour, sugar, cinnamon, grated lemon rind and the rum and mix well. You can do this in a food processor. Beat the egg whites until stiff, fold in and pour into a buttered and floured 25cm cake tin. Bake in the oven at 180ºC/160ºC fan/gas 4 for about 40 minutes, or until it is firm. Serve hot or cold dusted with icing sugar.