Line slow cooker with aluminum foil collar, then line with foil sling and coat with vegetable oil spray.
Adjust oven rack to middle position and heat oven to 225°. Spread bread over rimmed baking sheet and bake, shaking pan occasionally, until dry and crisp, about 40 minutes. Let bread cool slightly, then transfer to very large bowl.
Mix chocolate chips into dried bread; transfer to prepared slow cooker.
Whisk cream, milk, egg yolks, Nutella, 3/4 cup granulated sugar, vanilla, and salt together in bowl, then pour mixture evenly over bread. Press gently on bread to submerge.
Mix remaining tablespoon granulated sugar with brown sugar then sprinkle over top of casserole. Cover and cook until center is set, about 4 hours on low. Let cool for 30 minutes before serving.
Serves 8 to 10