Place the butterscotch chips in a large bowl with the margarine. Microwave for 1 minute, stir the mixture and cook for another 30 seconds in the microwave. Stir until all the chips are melted.
Stir in the graham cracker crumbs and the chopped nuts. In a separate bowl using an electric mixer combine the cream cheese, egg, condensed milk, and vanilla until smooth.
In a 9 x 13-inch baking pan coated with cooking spray press in 1/2 of the graham cracker mixture. Pour the cream cheese mixture over the top, and then sprinkle in the other half of the graham cracker crumbs on top.
Bake for 25-30 minutes-chill and then cut into bars.
If you use frosting to make sure the bars are completely cooled before frosting them.