Grease a 9x13-inch baking dish. Add the cubed brioche bread and crushed vanilla wafers. Gently press the bread mixture down slightly. Arrange sliced bananas on top if desired.
In a large mixing bowl, whisk together the mashed bananas, eggs, egg yolks, melted butter, heavy cream, cinnamon, vanilla, salt, and brown sugar until smooth and fully combined.
Pour the custard evenly over the bread mixture, gently pressing the bread down to help absorb the liquid. Let soak for at least 1 hour or cover and refrigerate overnight for even more flavor.
Preheat oven to 375°F. Sprinkle the top generously with turbinado sugar.
Cover loosely with foil and bake for 20–25 minutes. Remove the foil and continue baking another 20–25 minutes, or until the top is golden brown and the center is just set.
Let cool for 10–15 minutes before serving. Top with whipped cream, caramel sauce, toasted coconut, or fresh bananas.