Instructions

• Grease a 9 x 13-inch (23 x 33 cm) baking dish with butter; set the dish aside.
• In a small saucepan over medium heat, combine ½ cup (1 stick) unsalted butter, ¾ cup packed light brown sugar, and 2 tablespoons light corn syrup. Whisk constantly until the mixture melts, becomes smooth, and just begins to bubble. Remove from heat and immediately pour the caramel into the prepared baking dish, tilting the dish to spread the caramel in an even layer across the bottom.
• Slice the brioche: if you like, trim off the crusts (this can be rough, it does not need to be perfect). Cut the bread into about 1½-inch (3.75 cm) thick slices. You may not need all the bread; set aside any extra.
• Arrange the brioche slices, flat side down, on top of the caramel in a single layer so the entire base of the pan is covered. Tear or trim slices as needed to fill gaps.
• In a medium bowl, whisk together 5 large eggs, 1½ cups half-and-half, 1 teaspoon vanilla extract, 3 tablespoons Grand Marnier, 1 teaspoon orange zest, and ¼ teaspoon salt until the mixture is smooth and uniform.
• Pour the egg-and-cream mixture evenly over the arranged bread. Press the bread slices down gently with a spatula or your hands so the custard soaks into the bread as much as possible.
• Cover the pan with plastic wrap and refrigerate for at least 20 minutes, or up to overnight, to allow the custard to fully soak into the bread.
• When ready to bake, remove the plastic wrap and place a rack in the middle of the oven. Preheat the oven to 350°F (175°C).
• Bake the casserole, uncovered, on the middle rack for 40 to 50 minutes, until the top is puffed and golden and the center is set (a knife inserted in the center should come out mostly clean).
• Let the casserole cool for a few minutes in the pan (it will deflate slightly), then serve hot—the caramel base will firm up as it cools, so it is best enjoyed warm.