If you can’t find pre-made bacon jam, following these steps for making your own:
Add chopped bacon to a large pot and cook on medium heat for 15-20 minutes until slightly crispy. Drain in a colander or on a paper towel-lined plate. Reserve 1 tablespoon of the bacon grease.
Add butter to pot and return to medium heat. Stir in the onion and salt and cook until softened, about 8 minutes. Stir in the water, maple syrup, vinegar, brown sugar and Dijon mustard. Bring to a boil, then reduce the heat to a simmer. Add the bacon back in. Cook, stirring occasionally, until thickened, about 30 minutes. If too dry, add more water and continue to simmer.
Spread ½ tablespoon mayo on each bread slice. Place a slice of cheddar cheese on the plain side of 4 slices of the bread. Spread some of the bacon jam on top of the cheese. Divide the Brie between the sandwiches and layer on top of the jam, followed by a slice of white American cheese. Top with remaining bread slices, buttered side up.
Heat a griddle over medium-low heat. Cook the sandwiches until golden brown and the cheese has melted.