Slice the challah 12+ hours before you begin this recipe, OR run each piece through the toaster briefly so it’s slightly dried but not browned. Arrange in a large baking dish, and set aside.
Whisk together the brown sugar, cocoa powder, cinnamon, and salt until no lumps remain.
In a separate bowl, whisk together the eggs, oat milk, and vanilla until smooth.
Combine the wet and dry ingredients, again whisking until smooth and combined. Pour this mixture over your prepared slices of bread. Cover and refrigerate for 1-2 hours (or longer), ideally periodically flipping the pieces so they can soak in the liquid from both sides.
When the bread is thoroughly soaked, layer it upright in a baking dish (I used a 9-inch pie pan). Sprinkle the chocolate and hazelnuts between pieces as you arrange them. Pour any remaining liquid over the top.
Loosely tent with foil, and bake at 350°F for 45 minutes, or until you can see the remaining liquid has set up and is not jiggling. Serve hot, dusted with confectioners' sugar (optional) and maple syrup.
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.