In a large bowl combine zwieback crumbs, marshmallows, nuts and brown sugar.
In a saucepan over low heat cook the evaporated milk and butterscotch chips. Stir mixture constantly until the chips have melted and the mixture is smooth. Reserve 1/2 cup of the butterscotch mixture for glaze. Combine the remainder with the dry mixture and mix well.
Spread batter into one 8 inch square buttered baking pan. Drizzle reserved butterscotch mixture over the top. Refrigerate bars for several hours or overnight before cutting and serving. Store leftovers in a covered container in the refrigerator.