Preheat the oven to 180C/350F/gas 4. Butter and flour an 18cm (7in.) round cake tin.
Beat the butter with the sugar until fluffy. Add the egg yolks, one by one. Beat in the zest and the ricotta. Sieve in the flour and the baking powder and mix until just combined.
In a separate bowl beat the egg whites to stiff peaks. Add a little to the ricotta mix to loosen it up, then fold the rest of the egg whites into the mix, taking care not to deflate them.
Spoon the cake mix into the prepared tin and bake for 45-50 minutes until it’s dark golden and firm to the touch.
