Melt the butter in a saucepan. Blend in the flour. Add the bouillon and salt. Stir and cook the mixture over moderate heat for about 2 minutes or until thickened. Stir in the mushrooms and cook for another minute. Remove from the heat.
Mix the egg yolk with the cream. Stir a small amount of the hot mixture into the egg yolk mixture. Stir the egg yolk mixture into the saucepan. Cook over low heat for 1 minute. Stir in the chicken or turkey.
Preheat the oven to 350°F.
In a large shallow baking dish, fill the crepes with the chicken mixture and fold the crepes. Sprinkle the crepes with cheese.