HEAT oven to 350°F. Arrange graham wafers in single layer in 15x10x1-inch pan.
BRING butter and sugar to boil in medium saucepan on medium heat; cook 2 min. or until butter is completely melted and mixture is well blended, stirring frequently. Pour over grahams; immediately spread to completely cover all grahams.
BAKE 6 to 8 min. or until sugar mixture is lightly browned and bubbly; top with chocolate. Bake 1 to 2 min. or until chocolate is melted. Remove from oven; immediately spread over grahams. Sprinkle with nuts; press lightly into chocolate with back of spoon. Cool completely before breaking into squares.
serving size = 1 square (29 g)