Preheat the oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish with some of the melted butter.
Arrange the thick bread slices in the prepared baking dish, overlapping them slightly if needed.
In a large bowl, whisk together the eggs, whole milk, granulated sugar, cinnamon, vanilla extract, and salt until smooth and evenly combined.
Pour the cinnamon custard evenly over the bread, making sure the slices are well coated.
Gently press the bread down with a spatula or the back of a spoon so it can absorb the custard.
Let the bread soak for 10–15 minutes.
Drizzle the remaining melted butter over the top and sprinkle with the optional brown sugar.
Bake uncovered for 35–40 minutes, or until the center is set and the top is lightly golden.
For a crispier top, broil for 1–2 minutes, watching carefully to prevent burning.
Remove from the oven and let the French toast rest for about 5 minutes.
Serve warm with maple syrup, powdered sugar, fresh berries, sliced bananas, or your preferred toppings.