Wash mushrooms and sauté whole (including stems) in 2 tablespoons of the butter. When browned, remove the mushrooms and mince. Stir the rest of the butter into the pan. Add the flour and stir until smooth. Add the milk a little at a time and mix well. Let cook until smooth and thickened, stirring constantly.
Just before removing from the heat, add the salt, beaten egg yolk, mushrooms and sherry. Stir thoroughly and heat for just 1 minute. Serve hot over fish, meats or fowl.