Prepare cupcakes according to the directions on the box. Let the cupcakes cool completely.
Use the opposite end of a piping tip to insert and twist into the center of a cupcake. Only go half way down. Remove the center of the cupcake making sure to save the center piece for later.
In a medium mixing bowl, use a hand held mixer to mix together the ingredients (marshmallow cream, butter and powdered sugar) for the cream filling.
Once the cream is light and fluffy, put it into a piping bag. Add the cream to the center of the cupcakes. Place the center of the cupcake back onto the cupcake.
In a large mixing bowl, add the softened butter for the frosting. Use a mixer on medium for 2 minutes.
Add the powdered sugar 1/2 cup at a time using the mixer to stir it together.
Add the banana extract and heavy cream, still using the mixer. If the frosting is too stiff add 1 tablespoon of water.
Frost each of the cupcakes, then sprinkle some of the Nilla Wafers on top.