Cream butter with egg and powdered sugar and mix with flour on board to a kneaded dough – pour dough into a prepared 26 springform pan.
Beat egg whites until firm, whip with half the amount of sugar- beat butter, egg yolks with remaining sugar until creamy, stir in remaining ingredients, finally fold in beaten egg whites – (if the buttermilk is very thin, then I recommend adding 2-3 tbsp.
semolina). Spread this mixture on the dough and bake in the preheated oven at 160° convection oven for about 65-70 minutes – cover with aluminum foil after 30-40 minutes – let cool in the pan.
The curd mass feels after this baking time still very soft, but is baked through – but only after a few hours then cut.