Instructions:
Prepare the Caramel (the Night Before): This dish is best prepared the night before! Grease a 9×13-inch baking dish. In a medium-sized saucepan, combine the 1 ½ cups dark brown sugar and 1 cup butter. Cook over medium heat for 5-6 minutes, stirring constantly, until the mixture is smooth and bubbling. Stir in the 1 teaspoon vanilla extract and remove from heat. Pour the hot caramel mixture into the prepared baking dish, spreading it evenly across the bottom.
Assemble the French Toast: Slice the 1 large loaf of French bread into slices about 1 ½ inches thick. Arrange the sliced bread in a single layer over the caramel mixture in the baking dish. In a medium-sized bowl, whisk together the 4 eggs, 2 cups milk, 2 teaspoons vanilla extract, and ½ teaspoon salt. Pour this custard mixture evenly over the bread slices. Gently press the bread down with a spatula to help it absorb the liquid.
Chill Overnight: Cover the baking dish tightly with plastic wrap and place it in the refrigerator. Let it chill overnight, or for a minimum of 5 hours, to allow the bread to soak up all the custard.
Bake (in the Morning): When you are ready to bake, preheat your oven to 350°F (175°C). Remove the plastic wrap. Drizzle the ¼ cup melted butter over the top of the dish. In a small bowl, mix together the 2 tablespoons granulated sugar and 1 teaspoon cinnamon, then sprinkle this cinnamon-sugar mixture evenly over the melted butter. Bake for 45-55 minutes, or until the top is golden brown and a knife inserted into the center comes out clean.
Serve: Dust the top generously with powdered sugar before serving. Enjoy the warm, sweet, and decadent French toast bake!