Crack the 3 eggs and extra yolk into a shallow dish. Add the milk, vanilla, cinnamon, sugar, and salt. Whisk for 60 seconds until no egg white streaks remain.
Place 2 bread slices flat in the custard. Soak 45 seconds on the first side, press firmly, flip, and soak 45 seconds on the second side with another press. Transfer to a cutting board and rest 1 to 2 minutes.
Heat a skillet over medium heat. Add 1 tbsp butter and wait for the foam to subside, about 90 seconds.
Place the 2 soaked slices in the pan. Cook 3 to 4 minutes undisturbed until deeply golden on the bottom and set at the edges.
Flip and cook 2 to 3 minutes until the second side is golden and the center springs back firmly when pressed.
Remove to a warm plate. Add the remaining 1 tbsp butter to the pan, wait for the foam to subside, then repeat with the remaining 2 soaked slices.
Serve immediately with maple syrup and a dusting of powdered sugar.