Bacon jam: cook half-inch sliced bacon strips in a medium saucepan over medium heat. Cook until bacon gets brown and crispy.
Remove bacon from the pan to a paper towel lined plate or bowl to drain. Reserve 1 tablespoon of the bacon fat in the pan.
Add onions and cook until they are very soft—about 10 minutes.
Add garlic, maple syrup, dark brown sugar, apple cider vinegar, chili powder, and black pepper to the pan with cooked onions. Stir to combine.
Add bacon back into the pan with all the other ingredients. Cook while stirring frequently for about 10 minutes or until everything is soft and has the consistency of jam.
Add finished and cooled bacon jam to a jar or covered container and store in the refrigerator for no more than a couple of weeks.
Sandwich assembly: spread peanut butter on one slice of bread and then top with 8 to 10 banana slices.
Top the banana slices with bacon jam and try to spread the jam around for an even layer. Top the bacon jam with the second slice of bread.
Place a medium pan or skillet over medium or just below medium heat.
Add a teaspoon of butter to the pan and when it has melted and is bubbly, place the sandwich on top of the bubbly butter.
Cook the first side of the sandwich for 2 to 3 minutes and then flip. Once flipped cook the second side for about the same amount of time. If both sides aren't sufficiently browned to your liking, flip and cook each side for an additional minute or two until browned and crispy.
Once browned, remove the sandwich and allow it to cool on a cooling rack for 2 minutes or so before serving.