In a saucepan melt the butter; reduce the heat to low add the soy sauce, brown sugar, mustard and water. Cook and stir until sugar is dissolved. Remove from heat and cool. When cool pour marinade in a large resealable storage bag, add ckicken wings, turning to coat. Place in refrigerator and marinate 2 hours, turning occasionally. When ready to bake pour wings and marinade in a 13x9x2 pan. Bake 350' for 45 minutes, or until no longer pink. Drain wings on paper towels or brown paper shopping bags.