PREHEAT oven to 375.

Over hot water, COMBINE butterscotch chips and butter; STIR until chips are melted and mixture is smooth.

TRANSFER to a large mixing bowl; STIR in graham cracker crumbs and walnuts with a fork until mixture resembles fine crumbs; RESERVE 2 cups crumb mixture for topping.

PRESS remaining mixture into 15 x 10-inch baking pan; BAKE 12 min.

In large mixing bowl, COMBINE cream cheese and sugar; BEAT until creamy.

ADD eggs, one at a time, beating well after each addition; BLEND in flour and vanilla.

POUR evenly over hot baked crust; SPRINKLE with reserved crumb mixture; BAKE 25 minutes.

When completely cool, CUT into bars; CHILL before serving.