Cut bread into 1-inch-thick slices (or less) and set on a baking sheet. Place in oven to dry out slightly while you continue.
Crack eggs into a large bowl or shallow dish. Whisk in the milk, maple syrup, vanilla, cinnamon, and salt. Don’t overbeat, but make sure the ingredients are well blended.
Remove bread from oven and transfer to a plate. Return empty baking sheet to oven to keep warm.
Place a large skillet over medium-low heat.
While skillet warms up, gently lower two or three slices of challah bread into the egg mixture. Turn over the bread after 10 to 15 seconds and soak another 10 seconds on the other side, or until they feel heavy (but not so wet that they fall apart).
Melt about 1 teaspoon butter in the hot skillet (or use spray oil). Gently place the soaked pieces in the skillet, and cook on the first side about 2 minutes, or until nicely browned. Flip over, and cook another 2 minutes, or until done.
When second side is almost done, place another two or three slices of bread in the egg mixture to begin soaking as previously described. Repeat until all slices have been cooked.
Place cooked slices on the warm baking sheet and keep warm in the oven.