Grease and line a 20cm baking tin with baking paper. Whisk together the egg yolks and icing sugar with an electric beater till thick, creamy and pale. Blitz the ricotta in a food processor and fold into the egg mixture, along with the lemon zest and vanilla. gently fold in the flour. In another bowl, whip the egg whites and caster sugar to stiff meringue using an electric beater and then fold into the cake mixture.. Carefully spoon the mixture into the prepared tin and bake for 35-40 minutes. Serve cool dusted with icing sugar.