Heat the oven to 375°F and grease a 9-by-13-inch baking dish.
Spread half the brioche in the dish, add 1 cup blueberries and repeat both layers.
Whisk the eggs, milk, maple syrup, lemon zest, lemon juice, vanilla, cinnamon and salt until smooth.
Pour the custard slowly over the bread, press the cubes down gently and let the dish stand for 20 minutes.
Dot the top with the butter and cover the dish with foil.
Bake for 30 minutes, uncover and bake for 15 to 20 minutes more, until puffed, golden and 160°F at the center.
Rest for 10 minutes before scooping.