Preheat a medium skillet over medium heat and cook bacon until crispy. Remove to a paper towel-lined plate to drain excess fat.
Generously butter a 9x13” casserole dish.
In a large mixing bowl, combine the eggs, milk, maple syrup, cinnamon and salt, and whisk until blended. Gently toss the toasted banana bread, bananas, walnuts and bacon in with the egg mixture. Pour the mixture into the casserole dish, cover with foil and place in the refrigerator overnight.
The next day, preheat the oven to 375ºF. Bake covered for 1 hour and 15 minutes, until puffed. Remove foil and bake an additional 15 minutes until golden brown.