Line the back of your slow cooker (the side opposite the control panel) with a piece of foil doubled over--this protects it from the extra-hot heating elements there and prevents the custard from burning. Also, line the rest of your slow cooker with foil and Pam (I used a slow cooker plastic bag.).
Heat oven to 225 degrees F and move rack to middle position. Spread bread cubes on rimmed baking sheet so that they are in a single layer and bake until dry/crisp, about 40 minutes. Toss them around halfway through.
Let them cool and then mix together with the chocolate chips in the slow cooker.
Whisk the cream, milk, egg yolks, Nutella, 3/4 cup granulated sugar, vanilla, and salt together in a large bowl.
Pour this over the bread cubes in the slow cooker, making sure to coat the top layer as you are pouring. Depending on the size of your slow cooker, you can push on top of the bread cubes with a spatula to evenly coat them in the custard.
Mix 1 tablespoon granulated sugar with 2 tablespoons light brown sugar and then sprinkle evenly over the top.
Cover and cook until set in the center, about 4 hours on low. Then let it cool for 30 minutes before serving. (I only let it set for about 15 minutes.).
You can serve it warm with ice cream or whipped cream.