Gather 2 slices of sandwich bread, 2 tablespoons peanut butter, and 2 tablespoons marshmallow fluff.
Place the two slices of bread flat on a clean surface, side by side.
Using a knife or spoon, spread 2 tablespoons peanut butter evenly over the surface of one slice of bread.
Using a clean knife or spoon, spread 2 tablespoons marshmallow fluff evenly over the surface of the other slice of bread.
Place the two slices together with the peanut butter and marshmallow fluff sides facing each other and press gently to seal.
If you like, cut the sandwich in half (diagonally or straight down the middle) and serve.
To pack for later, put the sandwich in its own compartment of a lunchbox with fruit, veggies, and your favorite sides; add an ice pack to the lunch bag if necessary.