If using an electric griddle, preheat it to 350°F. For stovetop, place a large skillet or cast iron pan over medium heat.
In a shallow dish or pie plate, whisk together the eggs, milk, vanilla extract, half of the cinnamon (3/4 tsp), and the salt until completely smooth.
Dip one slice of brioche into the custard mixture, allowing it to soak for 15-20 seconds per side. It should be fully coated but not falling apart. Place on a clean plate. Repeat with 4 slices.
Add the remaining cinnamon (3/4 tsp) to the custard and whisk to combine. Dip the remaining 4 slices of bread.
Melt about 1 tablespoon of butter on the hot griddle or skillet. Cook the French toast in batches (don't overcrowd) for 2-3 minutes per side, until deeply golden brown.
To keep warm, place cooked toast on a wire rack set over a baking sheet in a 200°F oven for up to 30 minutes while you finish the rest.
Serve immediately with maple syrup, powdered sugar, and berries.