For the buckwheat cake, cream butter with sugar and egg yolks. Add flour, nuts, baking powder and vanilla sugar and mix well to obtain a reasonably smooth dough.
Beat the egg whites until stiff and fold into the mixture. Put everything into a greased baking pan. When cooled, cut apart, spread with jam, sprinkle with powdered sugar if desired.
Bake in the oven for approx. 45 minutes at 180 °C top/bottom heat.