1. Cream butter, sugar, egg and vanilla in a bowl with a mixer (the mixture will look curdled).
2. Stir in graham cracker crumbs, flour and salt. Reserve 2/3 cup of this mixture for the topping.
3. Press remaining mixture into a 13 x 9 inch pan (layer will be thin). Bake at 350 F for 15 minutes or until golden around the edges
4. Top hot base with peanut butter cups, marshmallows, chocolate chips and press lightly to adhere. Sprinkle reserved crumb mixture in clumps over the top and press lightly.
5. Bake 18-20 minutes or until marshmallows are puffed and just beginning to turn golden.
6. Cool bars for at least 30 mins before cutting. To prevent the marshmallow from sticking, coat the blade of the knife with nonstick spray before and during cutting.