Heat oven to 180 degrees C and line the base and sides of a 20cm round cake tin with baking paper.
Place 100g of the castor sugar and the lemon zest in a food processor and blitz until finely ground.
Then beat the butter, vanilla and sugar/zest mix together for about 10 minutes until pale and creamy.
Add the egg yolks one at a time, beating the mixture well after each egg yolk is added.
Add the almond meal and beat to combine and then fold the ricotta through the almond mixture.
Beat the egg whites in a clean bowl until soft peaks form.
Gradually add the remaining sugar to the egg whites and whisk until stiff peaks have formed. Mix a third of the egg whites into the cake mixture, then gently fold in the rest of the egg white mixture.
Pour the mixture into the prepared cake tin, smooth the top and sprinkle with the almond flakes.
Bake for 40-45 minutes until cooked and firm to the touch.
Allow to cool completely in the cake tin before removing.
Dust with icing sugar to serve.