Thoroughly mix butter, shortening and sugar in medium bowl.
Blend in flour.
Spread evenly in (8 x 8-inch) baking dish.
Microwave 2 minutes, 30 seconds on HIGH, or until set, rotating dish 1/4 turn after each minute.
Immediately sprinkle chocolate pieces on crust.
Let stand 2 to 3 minutes until chocolate softens.
Sprinkle with nuts, if desired.
Cut into squares while warm.