Line 15x10x1 pan with aluminum foil. Set aside. Put butter in 2 quart pan. Melt over low heat; add sugar. Stir over moderate heat until mixture becomes pale caramel color and thermometer reaches 300 degrees (about 10 minutes). Remove from heat; spoon evenly in foil lined pan. Cool about 5 minutes. Scatter butterscotch pieces evenly over candy; as soon as they are soft, about 4 minutes, spread with back of teaspoon over entire surface. Sprinkle with nuts. Chill until firm. Break into pieces.