Preheat the oven to 350°F.
In a large mixing bowl, mash the bananas until smooth.
Add the melted butter, granulated sugar, brown sugar, egg, vanilla extract, and sour cream to the bananas. Whisk until combined.
Gradually fold the dry ingredients into the wet mixture, stirring gently until just combined. Avoid overmixing.
Fold in the chocolate chips.
Pour the batter into a parchment-lined loaf pan.
Bake for 50–55 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs.
Remove from the oven and cover the top with marshmallows.
Bake for 3 minutes and then broil for the last minute. Watch carefully so the marshmallows do not burn.
Add chocolate pieces and graham crackers.
Let it cool.
Slice.
Enjoy.