Wipe the mushrooms, trim the stems, and chop fine. Melt the butter and cook the mushrooms in it. Dissolve the flour and salt in the milk and cream. Add the mushroom mixture and cook until very thick, stirring constantly. Spread the mixture in a flat pan to cool. When cool, form into balls about the size of a walnut. Dip in the beaten egg, then in breadcrumbs, and again in the egg and breadcrumbs. Fry in deep fat (370 degrees F) until brown. Serve with meat. Smaller balls may be made to serve as appetizers.