Take three quarts of unshelled, young, tender green peas ; shell them carefully, and keep them wrapped up in a wet napkin until needed. Clean, drain, and tie up a lettuce-head; put it in a saucepan with the peas ; season with a pinch of salt ; cover with a glassful of water, and add a quarter of a pound of very good butter. After cooking for a quarter of an hour, remove the lettuce, and when ready to serve, thicken the peas with three spoonfuls of cream, diluted with one egg yolk, adding half a pinch of white pepper, and a spoonful of powdered sugar. Let all thicken together for five minutes, and serve immediately in a tureen.