In a large mixing bowl, whisk together 2 1/2 cups milk and 7 large eggs until smooth and frothy.
Whisk in 1/4 cup brown sugar, 1 tablespoon ground cinnamon, 2 teaspoons vanilla extract, and 1/2 teaspoon saltuntil well combined. Slowly whisk in 1/2 cup all-purpose flour until smooth.
Heat a lightly buttered griddle or 12-inch nonstick skillet over medium-high heat.
Working in batches, soak 16 (1-inch thick) slices sourdough bread in the egg mixture until thoroughly saturated but not falling apart, about 30 to 60 seconds. Lift each slice and allow the excess egg mixture to drip off, gently running the edge of the bread along the bowl or use your fingers to remove extra liquid.
Place soaked bread slices on the griddle and cook over medium-high heat for 2 to 3 minutes per side, or until golden brown.
Transfer cooked French toast to a plate. Repeat with the remaining slices.
Serve warm with your favorite toppings such as maple syrup, powdered sugar, or fresh berries.
Notes
To Freeze: cool french toast completely on a wire rack and then transfer the bread to a parchment paper lined baking sheet. French toast can be stacked with a sheet of parchment paper in between each layer. Freeze until solid, then transfer to a resealable plastic freezer bag for long term storage.
To Reheat: Microwave on high for about 1 minute per slice until warmed through OR toast in a toaster until heated through for a crispy exterior.