Combine margarine, shortening, peanut butter, milk, vanilla, and butter flavoring. Add confectioners' sugar and beat until creamy. Add more milk, if needed, 1 teaspoon at a time.
Spread icing over one layer of a yellow cake. Slice bananas on top. Place second layer on top and frost top and sides.
Peanut Butter and Banana Frosting · Dinner Match Lab | Dinner Match Lab