Adjust oven rack to the middle position; preheat oven to 425°.
Pulse the bread, butter, and 1/2 cup cheese in a food processor until coarsely ground.
Cook the bacon in a Dutch oven over medium heat until crisp, about 8 minutes.
Transfer the bacon to a paper towel-lined plate and pour off all but 1 tablespoon of fat.
Cook the onion until softened, about 5 minutes.
Add in the garlic and cayenne; cook until fragrant, about 30 seconds.
Stir in the sweet potatoes, cream, buttermilk, and salt; decrease heat to med-low and cook, covered, until the potatoes are just tender, about 30 minutes.
Off the heat, stir in the remaining 2 cups cheese, cooked bacon, and 4 of the scallions.
Transfer the mixture to a 13 x 9 inch baking dish; top with the bread crumb mixture.
Bake until the crumbs are golden brown, about 15 minutes.
Cool 10 minutes, then sprinkle with the remaining scallion.