Preheat your oven to 375 degrees F. Coat a large rimmed sheet pan (18x13 inches) generously with nonstick cooking spray, making sure to cover corners and sides.
In a large bowl, whisk together the eggs, whole milk, ground cinnamon, vanilla extract, and salt until completely smooth and well combined.
Pour the egg custard mixture directly into the prepared sheet pan, spreading it evenly across the bottom.
Lay the brioche slices in a single layer on top of the custard. Press each slice down gently so it absorbs the liquid. Let soak for 3 to 5 minutes.
Carefully flip each slice over and let the other side soak for another 3 to 5 minutes to ensure even absorption.
Bake in the preheated oven for 25 to 30 minutes, or until the French toast is golden brown on top with slightly crispy edges and the center feels set when gently touched.
Remove from oven and serve immediately with butter and warm maple syrup.