Preheat the oven to 160 C / Gas 3. Grease a round 20cm springform cake tin and line the bottom with baking paper.
Beat the butter with 150g caster sugar until creamy, about 3 minutes. Add the vanilla sugar or extract, lemon zest and lemon juice; mix well. Add egg yolks one at a time and mix constantly after each addition.
Fold ground almonds and ricotta into the cake mixture using a plastic spatula until well blended.
Beat egg whites in a clean bowl until soft peaks form. Gradually add 75g caster sugar and continue mixing until it is firmer.
Fold beaten egg whites into the cake mixture until well blended. Spoon the mixture into the greased cake tin and smooth out the top.
Sprinkle ground almonds on top and bake for 50 minutes on the middle shelf of the preheated oven. The centre of the cake will be a bit wobbly but light golden brown on the top and around the edges.
Let the cake cool completely in the tin. Dust with icing sugar and serve.