Place two ripe bananas, 2 cups milk, eggs, sugar, cinnamon, vanilla, and salt in a blender. Blend on high until combined.
Grease a 13x9 inch baking dish. Place the cubed bread in the dish and pour the blended mixture over the top. Use your hands to press down the bread if necessary to ensure it’s submerged in the liquid.
Refrigerate for 30 minutes or up to overnight.
Preheat the oven to 375°F.
Bake casserole for 30–35 minutes, or until the eggs are just set.
For the caramel sauce:
Meanwhile, place the brown sugar, ¼ cup milk, butter, vanilla, and salt in a small saucepan. Heat over medium heat for 3–4 minutes. Remove and let cool slightly.
To serve, cut the french toast casserole into squares. Slice the remaining bananas into ½ inch slices. Put a spoonful of caramel sauce on a plate, place the square of french toast casserole on top, add a few banana slices, and drizzle with additional caramel sauce.