1. Combine sugar, flour and salt. Cut in the margarine until the particles are the size of peas. Reserve 3/4 cup of this mixture for topping. 2. Stir baking soda into buttermilk; add to flour mixture along with unbeaten egg. Blend well. 3. Pour batter into a greased and floured 9 x 13 x 2-inch pan. Sprinkle the reserved topping on batter. Bake in preheated 350-degree oven for 35-45 minutes.