Let the berries thaw at room temperature for about 15 minutes. In the meantime melt butter and remove from the stove. Separate the eggs. Beat the egg yolks, 1 tbsp. water and sugar with the whisk of the hand mixer until creamy.
Add liquid butter while stirring. Add flour and salt and continue stirring for about 2 minutes. Add milk while stirring. Beat the egg whites until stiff and fold them into the dough with a whisk so that the egg whites are visible.
Spread the berries on the bottom of a baking pan lined with baking paper (approx. 30 cm long). Pour the dough evenly onto it. In a preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: see
manufacturer) bake for about 1 1/4 hours. Let the cake cool down first, then put it in a cool place for about 1 hour. Lift out of the tin, dust with icing sugar and cut into slices.