Knead the flour, sugar, butter, 1 egg and baking powder into a shortcrust pastry. Spread half of it in a 26 cm springform pan, shape the rest into a sausage and press it to the edge.
Separate the 4 eggs. Whisk the egg yolks with sour cream, sugar and custard powder. Beat the egg whites until stiff and fold carefully into the sour cream mass. Spread the mixture on the shortcrust pastry.
Bake the cake at 160 degrees for 1 hour on top / bottom heat.