In a small saucepan, heat raisins and whiskey until it simmers. Let it stand for 20 to 30 minutes or microwave for 20 to 30 seconds, then let it rest.
Slice day-old French bread into 1/2-inch cubes (about 8 cups).
In a large bowl, whisk together half-and-half, whole milk, eggs, brown sugar, vanilla extract, cinnamon, nutmeg, and a dash of salt.
Add bread cubes and soaked raisins with their liquid to the mixture. Stir to coat, cover with plastic wrap, and refrigerate for 30 minutes to 1 hour, stirring occasionally.
Preheat the oven to 325°F. Butter a 2 1/2-quart baking dish.
Spoon the bread mixture into the prepared baking dish, ensuring even distribution of raisins.
Bake for about 1 hour or until firm and golden brown. Cool on a rack.
In a medium saucepan, combine heavy cream, whole milk, and granulated sugar. Set aside.
In a small bowl, whisk together cornstarch and whiskey or bourbon.
In another bowl, whisk egg yolks.
Over medium heat, bring the cream mixture to a boil. Add the cornstarch mixture, stirring constantly. Reduce heat to low and cook for 3 minutes, stirring frequently.
Whisk about 1/2 cup of the hot mixture into the egg yolks to temper them. Pour the egg yolk mixture back into the saucepan and cook for an additional minute.
Whisk in a pinch of salt, butter, and vanilla extract (if using).
Transfer the sauce to a bowl. If not serving immediately, place plastic wrap directly on top to prevent a skin from forming. The sauce will thicken as it stands.
When ready to serve, spoon warm sauce over each serving of bread pudding and enjoy.