To make the S'mores. Cut 4 thick slices of banana bread (you want them thick because the bread will break if it is too thin), spread a tablespoon of peanut butter on the inside of 2 of the slices of banana bread. Place two squares of chocolate on top, then place a marshmallow on top of that. Place the other slice of banana bread on top.
Heat a small pan over medium heat. Add a little butter and place the S'more in the pan. Cook for about 3-4 minutes and then very carefully flip the S'more (be as careful as you can, the bread is very moist so it rips very easily) and cook another 3-4 minutes or until most of the chocolate has melted and the marshmallows are getting gooey. Very, very carefully remove from the pan and allow the S'more to sit for a minute or two. If the S'more tears, dont freak. It is still insanely delicious! The S'mores will be gooey, hot and messy. This is one you're gonna want to eat with a fork!
To make the vanilla marshmallows place the gelatin in a small bowl and stir in the 8 tablespoons of cold water. Set aside. Spray an 8x8 pan with nonstick spray, then sprinkle some powdered sugar in the pan and shake well, dispersing it all over the pan, covering the entire thing.
In a small saucepan, combine the remaining cold water and sugar. Heat oven medium heat, stirring constantly until the sugar is dissolved and bubbly, about 3-4 minutes. Stir in the gelatin mixture, stirring, and bring it to a boil. Once boiling, immediately remove from heat. Add to the bowl of an electric mixer attached with a whisk. Let sit and slightly cool. Add in vanilla extract, and salt. Beat on medium-high speed for 10-15 minutes, until white and glossy and shiny and thick. Spread in the 8x8 pan and top with powdered sugar if desired. Let sit for about 4 hours, or overnight.
To cut, I run a knife along the outside of the pan, then turn it over and shake the marshmallow cube out onto a greased suface (I did not do this and half the marshmallows stuck to my counter. Using a knife sprayed with nonstick spray, slice the marshmallows into your desired size.