Make Cake: Preheat oven to 325º (300º if using dark pans). Butter and lightly flour 9 x 13 inch pan. Mix all ingredients together. Pour into prepared pan Bake 30 to 40 minutes. Add 1 package of marshmallows cut in half and placed on top of cake.
Make Icing: Mix sugar and cocoa, evaporated milk and vanilla and pour over marshmallows. Spread evenly over cake.