01 - Melt the butter in a small saucepan over medium heat. Add the brown sugar and corn syrup, stirring constantly until the mixture becomes smooth and bubbly. Pour the caramel into a 9x13-inch baking dish, spreading it evenly across the bottom.
02 - Place the bread slices in a single layer over the caramel, arranging them so they fit snugly. Slight overlapping is acceptable if needed to fit all pieces.
03 - Whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, and salt in a large mixing bowl until completely smooth and well combined.
04 - Pour the custard mixture evenly over all the bread slices. Gently press down on the bread with your hands or a spatula to help it absorb the liquid.
05 - Cover the baking dish tightly with plastic wrap and refrigerate for 8 to 12 hours to allow the bread to fully absorb the custard.
06 - Remove the dish from the refrigerator and let it sit at room temperature while the oven preheats to 350°F.
07 - Bake uncovered for 30 to 35 minutes, or until the French toast is puffed, golden brown on top, and the custard is completely set.
08 - Remove the baking dish from the oven and let it cool for 5 minutes before proceeding.
09 - Sprinkle the 1/4 cup granulated sugar evenly over the entire surface of the baked French toast. Using a kitchen torch, melt and caramelize the sugar until it forms a crisp, golden crust. Alternatively, place under the broiler for 1 to 2 minutes, watching closely to prevent burning.
10 - Serve warm, spooning any remaining caramel sauce from the bottom of the dish over each portion.