Let the berries thaw for about 15 minutes. Grease the bottom of the box cake tin (11 x 30 cm, approx. 1.8 l capacity) and line with baking paper. Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer).
Meanwhile melt butter and remove from the stove. Separate eggs. Beat the egg yolks, 1 tbsp. water and sugar with the whisks of the mixer until creamy. Stir in liquid butter in a thin stream. Add flour and 1 pinch of salt. Continue stirring for about 2 minutes. Add milk while stirring. Beat the egg whites until stiff and fold them only briefly into the dough with a whisk (the dough must not be smooth and homogeneous. Egg whites should remain!)
Spread the berries on the bottom of the mould and place the dough on top. Bake in a hot oven for about 1 1⁄4 hours. Remove the cake, let it cool down for about 30 minutes, then put it in a cool place for about 1 hour. Lift out of the mould. Dust with icing sugar and cut into slices.