This is very tasty fluffy cake.
Melt the butter and leave to cool, set aside. Beat the eggs, sugar, vanilla sugar with electric mixer until the mixute becomes very fluffy. Slowly add the sifted flours and mix very gently, then add the lemon zest.
Without interrupting mixing slowly pour in the melted butter and lemon juice. Mix well very gently. Pour the mixture into greased loaf tin and bake in the preheated oven to 170C for 60 minutes or until a skewer comes out clean when inserted into the centre of the cake. Allow to cool for 15 minutes in the tin before turning out onto a wire rack.