Preheat oven to 375 degrees F. Lightly butter a 9-inch square baking dish.
In a large bowl, cream butter and brown sugar until light and fluffy. Add flour and graham cracker crumbs; stir until well blended. Press mixture onto bottom of the prepared baking dish. Sprinkle over the top with marshmallows, chocolate chips, and walnuts.
Bake 15 to 20 minutes or until golden brown. remove from oven and cool on a wire rack. Cut into bars and remove from the baking dish.
Yields 3 dozen bars.