Step 1Cut the brioche into cubes. Spread out and set aside to dry out overnight.
Step 2In a deep bowl, combine the cream, milk, eggs, egg yolks, vanilla seeds and sugar. Using a handheld immersion blender, mix everything together. (Alternatively, blend the liquids together, in batches if necessary, using a stand blender.)
Step 3In a separate bowl, slice the bananas into 1/4-inch thick coins. Combine the slices with the chopped chocolate and brioche, and gently press into a 5-quart gratin or baking dish. Add enough liquid to cover the bread. Place in the refrigerator to soak overnight, and refrigerate the rest of the liquid.
Step 4Heat the oven to 350 degrees. Remove the dish from the refrigerator and add the remaining liquid to the bread mix.
Step 5Sprinkle the top with demerara sugar and bake until puffed and set, about 1 hour.