Using a hand-held mixer, combine the sugar, margarine, and vanilla in a large bowl. Add the banana, stirring well to combine.
In a separate bowl, combine the flour, cocoa, baking soda, and salt, stirring well with a whisk. Alternating with the soy milk, add the flour mixture to the sugar mixture a little bit at a time, beginning and ending with the flour mixture and mixing after each addition.
Spoon into the paper liners. Bake for 20 minutes, or until the cupcakes spring back easily when touched lightly in the center.
Allow to cool completely before frosting.
Using a hand-held mixer, combine all the ingredients in a bowl. Spread onto the cooled cupcakes.Servings: approximately 1 dozen