Arrange a rack in the middle of the oven and heat to 300°F.
Fit a wire rack inside a rimmed baking sheet. Place the bread in a single layer on the rack. Bake, flipping halfway through, until dry to the touch on both sides, about 15 minutes total. Remove from oven and let cool 5 minutes.
Meanwhile, make the custard. Place 1 ½ cups milk or cream, 6 large eggs, 2 tablespoons maple syrup, 1 tablespoon vanilla extract, ½ teaspoon ground cinnamon, and ¼ teaspoon salt in a quart-sized liquid measuring cup or medium bowl. Whisk until fully combined (no streaks of egg remaining). Pour into a 9x13-inch baking dish.
Add as many slices of challah as can fit in a single layer. Soak, flipping once, until drenched but not falling apart, 1 to 2 minutes per side. Return to wire rack (letting excess drip off into baking sheet) and repeat with remaining challah slices.
Melt 2 tablespoons of the butter in a large nonstick skillet over medium heat. When the foaming subsides and the butter is sizzling but not brown, add 4 soaked challah slices. Cook until the bottoms are golden-brown and crispy, 3 to 4 minutes. Flip and cook until the second side is browned, about 3 minutes more. Add remaining 1 tablespoon butter to the pan in between batches, swirling as it melts to ensure it doesn’t burn.
Serve warm with more maple syrup.
