Roughly chop the chocolate and melt over a warm water bath. Coarsely chop the nuts. Drain cherries well in a sieve.
Separate eggs. Beat egg whites until stiff. Cream butter, sugar, vanilla sugar, salt and lemon zest with the whisk of the hand mixer. Stir in the egg yolks one after the other. Sift flour, baking powder and starch.
Alternately stir yoghurt and milk into the butter egg mixture. Fold in chocolate and nuts. Fold in the beaten egg white.
Line a square springform pan (24 x 24 cm) with baking paper. Fill in the dough and smooth it down. Spread cherries on the dough. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see
manufacturer) Bake for 55-60 minutes. Remove from the oven and allow to cool. Remove from the tin, dust with icing sugar and cut into pieces. Whipped cream tastes good with it.