In a saucepan, melt the butter with brown sugar and corn syrup, stirring till smooth and the sugar is dissolved. Pour into a 9 x 9-inch baking dish.
Cut one-inch slices of brioche or challah bread and arrange them in one layer over the caramel, cutting a slice if needed to cover completely.
Whisk together eggs, then add half and half, vanilla, Grand Marnier (if using), and a pinch of salt. Pour over bread, then cover and refrigerate overnight.
The next morning, bring your French toast out of the fridge and allow it to sit at room temperature for about an hour. Preheat the oven to 350ºF (175ºC).
Bake, covered, till puffed and golden, about 25-30 minutes.
To serve, flip slices so the caramel layer is on top. Serve with berries and real maple syrup if desired.
