In a large bowl, whisk together egg, milk, vanilla, cinnamon, and sugar. Set aside.
Heat a large nonstick pan over medium low heat.
Add 1 Tablespoon butter.
Dip challah, one piece at a time, in milk mixture, turning to coat completely.
Place in pan.
Cook until browned, about 1-2 minutes.
Use a spatula to flip.
Cook until brown, about 1-2 minutes.
Remove to serving tray.
Repeat with each slice of challah, adding more butter to the pan as necessary.
Serve with powdered sugar and/or syrup.