In a small saucepan over medium heat, combine the butter, brown sugar, and corn syrup. Stir constantly for 3-4 minutes until smooth, bubbling gently, and glossy. Remove from heat, stir in the maple syrup, then pour immediately into the bottom of a 9x13 inch baking dish. Spread evenly with a spatula.
Slice the brioche loaf into 3/4 to 1 inch (2-2.5cm) thick slices. Arrange them in a single layer over the caramel base in the baking dish, fitting them snugly together. Gently press each slice down so it makes contact with the caramel.
In a large mixing bowl, whisk the eggs until combined. Add the half-and-half, vanilla extract, Grand Marnier (if using), salt, and cinnamon. Whisk vigorously until the mixture is smooth and fragrant.
Pour the custard evenly over the bread in the baking dish, using a spatula to gently press the tops so the bread absorbs the liquid. Cover tightly with plastic wrap or foil and refrigerate for a minimum of 4 hours, or overnight (8-10 hours) for best results.
When ready to bake, remove the dish from the refrigerator and let it sit at room temperature for 15-20 minutes. Preheat the oven to 350°F (175°C). Remove the cover and bake uncovered for 35-40 minutes, until the top is golden brown and puffed and the custard is set. Tent with foil if browning too quickly.
Remove from the oven and let cool for 5 minutes. Run a spatula around the edges to loosen, then scoop individual portions out of the dish, flipping each piece so the caramelized bottom faces upward. Dust with powdered sugar and serve with fresh berries, extra maple syrup, or whipped cream.