Liberally grease a ring tin with about 20g butter. Sprinkle evenly with the breadcrumbs and set aside. Preheat oven to 180 degrees C.
Sift together the plain flour and baking powder. Stir in the potato flour. Set aside.
In a clean bowl, cream the butter till light and fluffy. Whisk in icing sugar, vanilla sugar, egg yolks, sour cream, almond extract, lemon zest and juice. Beat till combined. Add in the dry mixture and stir till well incorporated, then fold in the raisins.
In a clean bowl, beat the egg whites to stiff peaks. Gradually add the egg whites to the cake mixture, gently folding in till evenly incorporated. Spoon the mixture into the prepared tin.
Bake for 30 to 40 minutes, or until a skewer inserted in the centre of the cake comes out clean. Cool slightly before inverting the cake onto a serving plate, and dust with icing sugar if desired.