Place 1 16-ounce loaf sourdough bread of sourdough bread on cutting board. Slice loaf into 1-inch thick bread slices.
Once entire loaf is sliced, stack 3 to 4 bread slices on top of each other. Slice stack of bread in half. Rotate stack 90°, then slice stack into 1-inch wide strips. Repeat until all sourdough has been sliced into strips.
Spray baking dish lightly with cooking spray, making sure to cover bottom, sides, and inner corners of dish.
Scatter 3 tablespoons butter (cubed) evenly across bottom of baking dish. Place strips of bread in dish, arranging bread in rows on top of butter, with strips of bread slightly overlapping.
Slide slices of 4 medium bananas between strips of bread, distributing banana slices evenly throughout dish.
Crack 8 large eggs directly into medium mixing bowl and whisk eggs vigorously until fully combined. Mixture should be pale in color with no streaks of egg remaining.
Add 2 cups milk, ½ cup heavy whipping cream, ½ cup granulated white sugar, ¼ teaspoon cinnamon, and 2 teaspoons pure vanilla extract to bowl with eggs. Whisk vigorously until ingredients are fully incorporated.
Pour custard mixture into baking dish, covering sourdough completely and evenly. Cover baking dish with aluminum foil and place dish in refrigerator. Chill casserole at least 2 hours, and no more than 12 hours.