Preheat the oven to 170C/325F/Gas 3. Lightly grease a baking ring mould.
Cream together the butter and sugar in a bowl until the mixture is pale and creamy.
Stir in the egg yolks, plain flour, potato flour or cornflour and lemon zest together with four tablespoons of milk. Stir in 1-2 more tablespoons of milk if the mixture is very thick. It should be creamy, light and stand in soft peaks in the bowl.
Stir in the pane di angeli, or the baking powder and vanilla essence mix.
Whisk the egg whites until soft peaks form. Then fold the egg whites into the mixture.
Pour the mixture into the greased baking ring mould and bake in the oven for 20 minutes or until golden brown and springy to the touch. Serve.
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