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