Arrange graham squares in single layer in 15x10x1-inch pan sprayed with cooking spray.
Bring butter and sugar to boil in medium saucepan on medium heat; cook 1 to 2 min. or until butter is completely melted and mixture is well blended, stirring frequently. Pour over grahams; immediately spread to completely cover grahams.
Bake 6 to 8 min. or until topping is lightly browned and bubbly. Top with chopped chocolate; bake 1 to 2 min. or until melted. Immediately spread chocolate over grahams. Sprinkle with nuts; press lightly into chocolate with back of spoon. Cool completely before breaking into squares.