Combine egg whites, sugar, water, vanilla, and corn syrup iin bowl of standing mixer or large heatproof bowl and set over medium saucepan filled with 1 inch of barely simmering water (do not let bottom of bowl touch water).
Cook, stirring constantly, until sugar is dissolved and mixture registers 160 degrees on instant-read thermometer, 5 to 10 minutes.
Remove bowl from heat and transfer mixture to standing mixer fitted with whisk attachment. Beat on medium speed until soft peaks form, about 5 minutes. Increase speed to medium-high and continue to beat until mixture has cooled to room temperature and stiff peaks form, 5 minutes longer.
To make grilled Fluffer-Nutell-ers, spread a 1-2 tablespoons fluff each on half of the pumpkin bread slices, then spread 1-2 tablespoons of Nutella on each of half of the other slices. Sandwich the fluff and Nutella smeared pieces together.
Heat the butter over a large skillet over medium. Add sandwiches to the pan, two at a time, and heat on each side 2-3 minutes, or until browned and crisp outside. Remove sandwiches from pan, let cool for a minute or two, and garnish with powdered sugar before diving in.
Recipe by Allons-Eat! at https://www.allonseat.com/marshmallow-fluff/