Heat 1 cup of heavy cream and 2½ cups of whole milk in a saucepan until simmering. Pour over 225g of coarsely chopped semisweet chocolate in a large bowl, stir until melted, and cool slightly.
Whisk in 4 large eggs, ½ cup of sugar, 2 teaspoons of vanilla extract, and ½ teaspoon of salt.
Stir 10 cups of cubed day-old challah (1in / 2.5cm cubes) and 1 cup of semisweet chocolate chips into half of the custard. Spread in a greased 9 × 13in / 23 × 33cm baking pan and pour over the remaining custard.
Cover and let soak for at least 30 minutes. Meanwhile, preheat the oven to 350°F / 175°C.
Bake for 45-50 minutes, covered with aluminum foil for the first 20-25 minutes, until the top is crisp and a toothpick comes out with a few moist crumbs.
