Grease a 9-by-13-inch baking dish and spread the brioche cubes in an even layer.
Whisk the eggs, milk, cream, 1/4 cup brown sugar, vanilla, 1 tsp cinnamon and salt until no egg streaks remain.
Pour the custard over the bread, press the cubes down gently and refrigerate the covered dish for at least 4 hours or overnight.
Heat the oven to 350°F and let the dish stand at room temperature for 20 minutes.
Rub the flour, pecans, remaining brown sugar, remaining cinnamon and cold butter together until uneven crumbs form, then scatter them over the bread.
Bake uncovered until the center reaches 165°F and no liquid custard appears when pressed, 40 to 45 minutes.
Rest for 10 minutes and drizzle with the maple syrup before cutting.