Crack eggs into a large shallow dish (9x13 baking dish works well). Add the milk, sugar, vanilla, cinnamon, nutmeg if using, and a pinch of salt.
Whisk everything together really well for about 30 seconds until completely combined with no streaks of egg white and sugar is dissolved.
Turn Blackstone burners to low-medium and let the griddle preheat for about 10 minutes until surface temp reaches 300 to 325 degrees F.
Add about 2 tablespoons of butter to the preheated griddle, spreading it around with a spatula to coat the cooking area. Butter should melt and foam but not brown immediately.
Take a slice of bread and lay it in the egg mixture. Let it soak for about 5 seconds, then flip and soak the other side for another 5 seconds. Adjust timing based on bread thickness.
Place soaked bread directly on the buttered griddle, leaving space between slices.
Cook for 3 to 4 minutes on the first side until deep golden brown and edges look set.
Flip and cook another 3 to 4 minutes on the second side until golden brown and center feels firm when pressed gently.
Move finished slices to a cooler zone on the griddle to keep warm, or transfer to a baking sheet in a 200-degree oven.
Add more butter to the griddle before each new batch and repeat until all slices are cooked.