Butter and flour a 21cm (8 1/2 1nch) cake tin and line the base with a disc of parchment paper.
Beat the butter and sugar until pale light and fluffy. Add the lemon zest, vanilla, egg yolks and ricotta and mix gently. Mix in the sieved flour and baking powder. Whisk the egg whites to stiffish peaks and fold into the ricotta mixture.
Place the mixture in the prepared tin and smooth the surface.
Bake for 35 minutes until risen, firm and golden in colour. Allow to cool on a wire rack for 10 minutes.
Remove from the tin and peel off the parchment paper.