Preheat the oven to 350 degrees F. Pour boiling water over instant oatmeal and let stand for 20 minutes. Cream together the margarine, brown sugar, and sugar. Next add the eggs, vanilla, and oatmeal and mix well. Blend in flour, soda, cinnamon, and salt. Mix well. Pour into 9 by 11-inch pan. Bake for 35 minutes.
Combine all ingredients. After the cake is cooked, spread icing over the cake. Place under broiler for 3 minutes or until frosting is slightly browned.