Rinse the crabs and drain very well. If necessary, dab dry with kitchen paper. Sprinkle with cognac and lemon juice, let it steep for a short time. Whip cream until semi-stiff. Mix with horseradish, mustard powder, sugar and tomato ketchup.
Season to taste with salt and pepper. Add the prawns. Clean, wash and shake the salad well dry. Line 4 bowls with it. Arrange the shrimp cocktail on top and grind pepper over it.