Prepare the Caramel Sauce:
In a small saucepan over medium heat, melt the butter. Add brown sugar and corn syrup, stirring until smooth and bubbly, about 3-4 minutes.
Pour the caramel sauce into a greased 9x13-inch baking dish, spreading it evenly over the bottom.
Prepare the French Toast:
Arrange the bread slices over the caramel sauce in a single layer, slightly overlapping if needed.
In a large mixing bowl, whisk together the eggs, milk, heavy cream, granulated sugar, vanilla extract, cinnamon, and salt until smooth.
Pour the custard mixture evenly over the bread, ensuring all pieces are soaked.
Cover the dish tightly with plastic wrap or foil and refrigerate overnight (or at least 4 hours).
Bake the French Toast:
Preheat the oven to 350°F (175°C). Remove the dish from the fridge and let it sit at room temperature for 15-20 minutes.
Bake, uncovered, for 35-40 minutes, or until the top is golden brown and the custard is set.
Serve:
Let the dish cool for 5 minutes before serving. To plate, use a spatula to scoop slices, flipping them upside down so the caramel sauce is on top.
Dust with powdered sugar, add fresh berries, or drizzle with maple syrup if desired.