Light grill and set for two-zone medium heat, establishing temperature at 325°F as indicated by a reliable leave-in thermometer.
Place each banana in an aluminum foil boat for stability, ensuring the tip and the stem are pointing up. Then, leaving the banana in the peel, slice down the length of the banana, like a hot dog bun. Don’t cut completely in half.
Pry apart the bananas from the cut side and evenly distribute brown sugar inside each one.
On top of the brown sugar, evenly distribute the peanut butter chips.
On top of peanut butter chips, place the mini marshmallows.
Place bananas (in the foil boat) on the grill grate over indirect heat, close the lid and let them bake for ten minutes or until peanut butter chips have melted and marshmallows are toasted.