Place the egg yolks in a large bowl and add the cinnamon and Marsala. Mix very well with a wooden spoon, then add the cold broth and mix thoroughly. Pass the ingredients of the bowl through a piece of cheese-cloth into a large flameproof casserole and add the salt and butter.

Warm 6 terra-cotta soup bowls in the oven.

Place the casserole over low heat and stir continuously until the moment before the broth reaches the boiling point; do not allow it to boil. Remove immediately from the heat and ladle into the individual heated soup bowls. Sprinkle each bowl with 1/2 teaspoon sugar and a "smell" (less than a pinch) of freshly grated nutmeg. Serve immediately.