Grease a baking dish or heat a griddle with butter.

Arrange the thick brioche or challah slices in the dish or lay them out for soaking.

In a large bowl, whisk together the eggs, heavy cream, whole milk, ½ cup granulated sugar, vanilla extract, cinnamon, and salt until well combined.

Pour the custard mixture over the bread slices, pressing gently so each piece absorbs the liquid. Let the bread soak for at least 30 minutes, or refrigerate and soak overnight for deeper flavor.

If baking: Preheat the oven to 350°F (175°C). Place the soaked bread in a single layer in the prepared baking dish. Bake until the custard is set and the bread is golden, about 30–45 minutes.

If using a griddle: Cook each soaked slice over medium heat for about 3–4 minutes per side until golden brown and cooked through.

Sprinkle a thin, even layer of the remaining granulated sugar over the surface of each cooked slice.

Use a kitchen torch to caramelize the sugar until it forms a crisp, golden crust. If you don’t have a torch, place the slices under a hot broiler for 1–2 minutes until the sugar bubbles and browns.

Let the French toast cool briefly so the caramelized top hardens. Serve warm with your favorite toppings.