Bring the chicken stock, prepared and measured out the day before, to the boil and simmer gently over reduced heat for a few minutes.
Meanwhile, beat the egg yolks in a large baking bowl until creamy. Add juice of one lemon, zest, salt and cayenne pepper and fold in Krem Double.
Next, leisurely fold small amounts of the hot stock into the egg mixture, beating continuously (about 2 ladles). Stir the resulting egg-lemon mixture leisurely into the chicken stock form. Transfer the soup to a cold baking dish and chill on ice.
Chill in the refrigerator for a few hours.
Chop the fish fillet into small cubes, season lightly with lemon, pepper and salt and mix with a small amount of egg white. Put this filling by the teaspoonful on each wan-tan sheet, brush the edge with egg yolk and place a second sheet on top. Use a round glass to cut out the shape.
Just before serving, cook the fish pockets in hot salted water and bring to the table on the spot in the cold soup.