Preheat a waffle iron according to manufacturer's instructions and spray with cooking spray.
Whisk milk, eggs, maple syrup, vanilla extract, and salt together in a wide bowl until thoroughly combined. Dip bread slices 1 at a time in the egg mixture, coating both sides completely. Lift bread with a slotted spatula to allow excess egg mixture to drain back into the bowl. Place dipped bread slices on a rimmed baking sheet and let rest until mixture soaks in, about 2 minutes.
Place dipped bread in the preheated waffle iron. Gently close the lid without forcing it down. Cook according to manufacturer's instructions until golden brown, 3 to 5 minutes. Repeat with remaining slices.
