01 - Preheat oven to 350°F and generously butter a 9x13-inch baking dish to prevent sticking.
02 - Spread bread cubes evenly across the prepared dish. Sprinkle raisins over the bread if using.
03 - Whisk together milk, heavy cream, eggs, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt in a large bowl until fully combined.
04 - Pour custard mixture evenly over bread cubes, pressing down gently to ensure thorough absorption. Let stand for 10 minutes.
05 - Drizzle melted butter over the soaked bread cubes.
06 - Bake for 40 to 45 minutes until golden brown and set in the center.
07 - Combine milk, heavy cream, sugar, and butter in a small saucepan. Heat over medium heat, stirring constantly until sugar dissolves and mixture is hot but not boiling.
08 - Mix cornstarch and cold water in a small bowl to form a slurry. Stir into the hot cream mixture and cook for 2 to 3 minutes, stirring constantly until thickened.
09 - Remove from heat and stir in vanilla extract and salt.
10 - Serve warm bread pudding topped with generous spoonfuls of warm vanilla sauce.