Cake: Preheat oven to 350 degree. Grease and flour a 9x13 pan. Cream margarine and sugar, add eggs and beat until fluffy. Add bananas. Mix dry ingredients in a separate bowl. Add buttermilk, then add dry ingredients. Beat on low speed until combined, then on high speed for 3 minutes. Pour into pan. Bake 45 - 50 minutes. Cool.
Frosting: While cake is baking, mix sugar and cocoa in 2-qt saucepan. Stir in milk, margarine and corn syrup. Heat to boiling, stirring often. Boil for 3 minutes, stirring often. Cool. Stir in vanilla and sugar. Frost cake.