HEAT oven to 400ºF.
PLACE crackers in single layer in foil-lined 15x10x1-inch pan.
COOK butter and sugar in saucepan on medium-high heat until butter is melted and mixture is well blended, stirring occasionally. Bring to boil; cook 3 min. without stirring. Spread onto crackers.
BAKE 5 to 7 min. or until topping is golden brown. Immediately sprinkle with chocolate; let stand 5 min. or until chocolate is softened. Spread chocolate over ingredients in pan; sprinkle with nuts. Cool. Break into pieces.