Heat oven to 180 C / Gas 4. Grease a 18cm round cake tin.
Beat the butter and sugar until light and fluffy, then add in the zest, eggs yolks and ricotta. In a separate bowl, whisk the egg whites to stiff peaks, then fold into the ricotta mixture. Finally, fold in the flour and baking powder then pour into the tin.
Bake for 35 minutes until risen, firm and golden. Cool for an hour in the tin, then slice and serve.
If you want to make this extra decadent, boil the juice of the lemons and orange, with 50g sugar until thick and syrupy (2 to 3 minutes), then pour over the cake to make it a drizzle cake!