Prep Pan & Bread: Grease a 9x13-inch ceramic or glass baking dish generously with softened butter. Arrange the cubed brioche bread evenly in the dish, gently pressing down so it forms an even layer.

Whisk Custard: In a large bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt until completely smooth and frothy.

Pour & Soak: Slowly pour the custard mixture evenly over the cubed brioche, ensuring every piece of bread is moistened. Use your hands or a spatula to gently press down on the bread cubes so they soak up the custard. (At this stage, you can cover and refrigerate overnight for up to 12 hours, or bake after a 20-minute soak).

Mix Cinnamon Streusel: In a medium bowl, combine the flour, brown sugar, cinnamon, and cold cubed butter. Use a pastry cutter or your fingers to work the butter into the dry ingredients until coarse, pea-sized crumbs form.

Bake to Golden Perfection: Preheat oven to 350°F (175°C). Sprinkle the cinnamon streusel evenly over the soaked brioche. Bake uncovered for 40 to 45 minutes until puffed, golden brown on top, and a knife inserted into the center comes out clean with no raw liquid custard.

Rest, Dust & Serve: Remove from the oven and allow to rest for 10 minutes to set. Dust with powdered sugar, cut into generous squares, and serve warm with melting butter and pure warm maple syrup!