Preheat your oven to 350°F (175°C).
Grease the 9x13 inch baking dish with the butter, ensuring the entire surface is coated.
Cut the raisin bread into 1-inch cubes and place them evenly in the greased baking dish.
In a large mixing bowl, whisk together the eggs, whole milk, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined.
Pour the egg mixture evenly over the cubed raisin bread, pressing down gently to ensure all the bread pieces are soaked.
If using, sprinkle the extra raisins over the top for added sweetness and texture.
Cover the baking dish with aluminum foil and refrigerate for at least 30 minutes, or overnight for best results.
Bake the casserole in the preheated oven for 30 minutes covered, then remove the foil and bake for an additional 15 minutes, until the top is golden brown and the center is set.
Remove from the oven and allow to cool for a few minutes before slicing and serving with maple syrup.