Heat oven to 350°F. Lightly grease and flour 8 or 9-inch square baking pan. For cake, pour boiling water over oats in medium bowl; mix well. In large bowl, beat margarine and sugars until well blended. Add egg whites and vanilla; beat well. Add reserved oat mixture and combined flour, baking soda, cinnamon, nutmeg and, if desired, salt; mix well. Pour batter into pan. Bake 55 to 65 minutes (8-inch pan) or 50 to 60 minutes (9-inch pan) or until wooden pick inserted in center comes out clean. Transfer cake in pan to wire rack. For topping, combine all ingredients in small bowl; mix well. Spread evenly over top of warm cake. Broil about 4 inches from heat 1 to 2 minutes or until topping is bubbly. Watch closely; topping burns easily. Cool cake in pan on wire rack. Store tightly covered at room temperature.