In a medium bowl, whisk the eggs until the egg whites have completely mixed with the egg yolks. Add the milk, sugar, vanilla extract, cinnamon, and salt and combine.
Pour the liquid mixture inside a 9x13 baking dish and place the sourdough slices in the dish. Lightly press down and move the bread slices around. Let the first side soak up the liquid for 3 minutes, flip them over, and let the sourdough soak up the liquid mixture for another 5 minutes until the liquid is nearly gone.
Cook
Heat a large nonstick or cast-iron pan over medium heat. Once hot, melt the butter and immediately place the soaked bread slices, usually 2 per batch, in place. Cook the first side for about 2 minutes.
Flip them over and continue cooking the other side for 2 more minutes, until the top of the middle feels puffy. Remove from the pan and keep warm while you cook the rest of them.
Serve
Serve warm, topped with a little butter and your favorite toppings and syrups.
Storage
Refrigerate cooked leftovers in an airtight container for up to 4 days.
Notes
Slice the sourdough bread between ½ to 1 inch thick. Use the middle slices, whenever possible, about 5 to 6 inches in length. The liquid mixture is calculated based on these measurements; but if you're using the smaller slices or the end of the sourdough round, you'll probably have enough liquid for one or two extra slices.