Heat the oven to 180C/fan.
and flour an 18cm spring-form cake tin.
Beat the butter and sugar with
an electric whisk until light and fluffy then beat in the zest, yolks and ricotta.
Whisk the egg whites to stiff peaks, then fold into the ricotta mixture.
Fold in the flour and baking powder then spoon the mixture into the tin.
Bake for 35 minutes until risen, firm and golden in colour.
Cool for an hour in the tin.