Heat the oven to 200°F (95°C) and set a wire rack over a sheet pan to keep finished toast warm.
In a medium bowl, whisk the eggs, granulated sugar, salt, and cinnamon until smooth. Whisk in the milk and vanilla until evenly combined.
Pour the custard into a shallow dish. Set the brioche slices nearby.
Heat a large nonstick skillet or griddle over medium heat. Add 1 tablespoon of the butter and let it melt, swirling to coat.
Working with 4 slices, dip each slice of brioche into the custard, about 10–20 seconds per side, until saturated but not falling apart. Lift and let excess drip back into the dish.
Lay the soaked slices in the skillet and cook until deep golden brown and the centers feel set, 2–3 minutes per side. Transfer to the rack in the warm oven.
Add the remaining 1 tablespoon butter to the skillet and repeat with the remaining 4 slices, dipping and cooking as in the previous step.
Serve hot with maple syrup, a dusting of powdered sugar, and fresh berries.