Grease a 9x13-inch baking dish. Arrange brioche cubes in the dish.
Whisk eggs, milk, cream, sugar, vanilla, cinnamon, nutmeg, and salt together until smooth.
Pour custard evenly over the bread. Press bread down with a spatula so all pieces contact the liquid. Cover and refrigerate at least 4 hours or overnight.
When ready to bake, preheat oven to 175°C (350°F). Mix brown sugar, flour, cinnamon, and salt in a bowl. Add cold butter and rub in with fingertips until crumbly. Scatter over the soaked bread.
Bake uncovered 40 to 45 minutes until the streusel is deep golden and a knife inserted in the center comes out without liquid custard. Internal temperature should reach 71°C (160°F).
Rest 10 minutes. Dust with powdered sugar and serve with maple syrup.