Using a 20cm spring form cake tin, either line it with baking paper or use a non stick spray.

Preheat your oven to 160ºC/ 320ºF.

Using your first 1/3 cup of castor sugar, add this to room temperature butter and beat .

Add the finely grated lemon and orange zests and vanilla and beat until light and creamy

Beat in the egg yolks and the ground almonds or almond flour depending on what you are using.

Add the Ricotta and combine gently.  This mixture will be thick.

Whisk your egg whites until they form stiff peaks and then add the second 1/3 of castor sugar and beat until it is glossy and firm.

Stir a large spoonful of the egg whites to the batter and loosen it up.  Then gently fold in  the remaining whites in batches until it is well combined.

Add to the tin and smooth the top of the batter, and scatter the slithered almonds on top.

Bake for 40-50 minutes until it is golden and firm. Check by inserting a skewer into the centre of the cake.  If it comes out clean it is done. 

Cool the cake completely before removing it from the tin.

Sprinkle with icing sugar and serve.