For the buttermilk ring cake, grease a ring cake pan and dust with breadcrumbs. Preheat the oven to 180 degrees. Mix buttermilk, egg yolks, oil and sugar until frothy.
Sift flour, cornstarch and baking powder and mix into the dough. Beat the egg whites and salt until stiff and fold in carefully, finally stir in the rum raisins.
Pour the dough into the cake pan and bake for about 50 minutes (test with a needle).