To Cook Prawns: Leaving the shells of the prawns intact, insert the toothpicks along the inside curves to prevent curling. Drop the prawn into 3/4 pt of boiling water and cook briskly, uncovered, for about 3 minutes until pink and firm. Drain in a sieve and cool quickly under cold running water. Remove the toothpicks and peel the prawns, leaving the last section of shell and tail on each one. De-vein the prawns by making a shallow incision across their tops with a small, sharp knife. Lift out the vein. Butterfly the prawns by cutting along their inner curves 3/4 of the way through, spreading the open and flattening with a cleaver.

Marinade

Put the vinegar, water, salt, sugar and Msg into a mixing bowl and stir together thoroughly. Add the prawns, marinade at room temperature for 1 hour.

Filling

Drop the egg yolks into a small saucepan and beat lightly with a fork. Remove 2 1/2 t of the yolks and set aside.

Add 3/4 pt of water to the remaining yolks, stir thoroughly, and cook over a moderate heat for 10-15 minutes until the yolks are firm and hard-boiled. Drain them in a sieve, and mash them into a paste. Beat the sugar, salt, salt, lemon juice, Msg, and the rest of the raw egg yolks and continue to beat. Rub the mixture through a fine sieve.

To Assemble And Serve Divide the seasoned egg filling into 6 equal parts and pack each part into the center of a prawn. Seal the edges by pressing them together. Serve at room temperature.

From "Recipes-The Cooking of Japan" from Time-Life International, c1965.