Preheat oven to 350 degrees F. Grease a 9x13 inch baking dish with butter.
Layer bread: spread bread cubes in the dish. Scatter raisins if using.
Make custard: whisk eggs, milk, cream, sugar, melted butter, vanilla, cinnamon, nutmeg, and salt until combined. Pour evenly over bread.
Soak: press bread down gently. Let soak 20 to 30 minutes, pressing occasionally.
Bake: bake uncovered 45 to 55 minutes until golden and center is set. Tent with foil if top browns too quickly.
Make vanilla sauce: melt butter in a small saucepan over low heat. Whisk in powdered sugar. Slowly whisk in beaten egg. Cook 2 to 3 minutes, stirring, until slightly thickened. Do not boil. Remove from heat, stir in vanilla and cream.
Serve: spoon warm vanilla sauce generously over each portion of warm bread pudding.