For the cupcakes:

Preheat oven to 350˚.
Prepare two 12 cupcake pans with liners.
In the bowl of a mixer, whip egg whites until stiff peaks form then set aside.
In a different bowl, whip butter until smooth and add sugar. Beat to blend.
Add mashed banana and mix well. It will look curdled.
Combine flour, baking powder, baking soda and salt. Add half the flour mixture to the banana mixture and beat until no flour remains visible.
Pour in buttermilk and egg yolks and mix well.
Add remaining flour mixture and beat well.
Fold in egg whites gently.
Fill cupcake liners about 2/3 full.
Bake for about 23-27 minutes or until cupcakes are set and golden browned on the top.
Remove from oven and cool completely.

For the filling:
Heat heavy cream to just below boiling and pour over chocolate.
Allow to sit for a few minutes then stir until it becomes smooth.
Poke a small hole in the center of each cupcake and fill with about 1 tsp of ganache.
You may have some left over.

For the frosting:
Melt the chocolate.
Combine all the ingredients except the sugar in the mixer and mix until smooth.
Add sugar, a little bit at a time until a smooth icing come together.
Frost cupcakes liberally.