Step 1: Prepare the Caramel. Preheat your oven to 350°F (175°C). In a medium saucepan over medium heat, melt the butter. Once melted, add the light brown sugar and corn syrup. Stir until the sugar is dissolved and the mixture is smooth. Allow it to simmer for about 2 minutes, then pour the caramel into the greased baking dish, spreading it evenly across the bottom.
Step 2: Prepare the Brioche. Cut the brioche into 1-inch cubes. Spread the brioche cubes evenly over the caramel layer in the baking dish.
Step 3: Make the Custard Mixture. In a large mixing bowl, whisk together the eggs, half-and-half, vanilla extract, Grand Marnier, orange zest, and salt until well combined.
Step 4: Assemble the Casserole. Pour the custard mixture evenly over the brioche cubes, ensuring that all pieces are soaked. Gently press down on the bread with a spatula to help it absorb the custard. Cover the dish with plastic wrap and refrigerate for at least 2 hours, or overnight for the best flavor.
Step 5: Bake the Casserole. When you’re ready to bake, remove the casserole from the refrigerator and let it sit at room temperature for about 15 minutes. Bake in the preheated oven for 45-55 minutes, or until the top is golden brown and a knife inserted into the center comes out clean. Allow it to cool slightly before serving.