Spray a 9x13-inch baking dish with cooking spray. Cut the brioche bread into 1-inch cubes and spread them evenly in the prepared dish. In a large bowl, whisk together the room temperature eggs, whole milk, heavy whipping cream, 3/4 cup packed light brown sugar, vanilla extract, 1 tsp ground cinnamon, and 1/2 tsp kosher salt until well combined. Pour this custard mixture evenly over the bread cubes, making sure all pieces are submerged or well-coated. I like to gently press down on the bread with the back of a spoon to help it absorb the custard more evenly.

In a separate bowl, combine 1/2 cup all-purpose flour, 1/2 cup packed light brown sugar, 1 tsp ground cinnamon, and 1/4 tsp kosher salt. Add the cold cubed unsalted butter and use a fork or pastry cutter to mix until the mixture resembles coarse breadcrumbs with pea-sized pieces of butter throughout. Cover the streusel mixture and refrigerate it alongside the casserole.

Cover the bread and custard mixture with plastic wrap or foil and refrigerate for at least 4 hours, or ideally overnight. This resting period allows the bread to fully absorb the custard and develop a creamy, custard-like texture throughout. The cold overnight soak is what transforms this into a proper French toast casserole rather than just bread soaked in egg.

Remove the casserole and streusel topping from the refrigerator and preheat your oven to 350°F. Let the casserole sit at room temperature for about 10 minutes while the oven heats. Evenly sprinkle the chilled streusel topping from Step 2 over the entire surface of the custard-soaked bread. Bake uncovered for 45 to 55 minutes, until the top is golden brown and a knife inserted into the center comes out clean or with just a few moist crumbs. I find that checking it around the 40-minute mark helps ensure it doesn't overbake—the top should be golden but not dark brown.

Remove the casserole from the oven and let it rest for 10 minutes before serving. This resting period allows the structure to set slightly, making it easier to portion and serve without falling apart. Dust the top generously with confectioners' sugar and drizzle with pure maple syrup just before serving.