Line cookies sheet with foil and grease with oleo. Lay one layer of saltines on cookie sheet (whole). Melt the 2 cubes of butter with white sugar and brown sugar. Stir constantly until it comes to a rolling boil, for 2 minutes. Pour liquid over crackers. Put pan in 425° oven for 4 to 6 minutes. (It will buckle and boil.) Remove from oven and scatter chocolate chips over all. As it melts, spread with a knife. Sprinkle chopped walnuts over all. Let cool slightly, then refrigerate. Take out and break into 1" to 2" chunks. Always keep in refrigerator, covered.