Heat 1/4 cup bourbon in a medium saucepan over medium-low heat until warm. Pour raisins into a small bowl. Pour warm bourbon over raisins; let stand at least 1 hour.
WHISKEY SAUCE: Bring 1 1/2 cups heavy cream to a simmer over medium heat in a small saucepan. In a small bowl, whisk together cornstarch and 2 tablespoons cold water; slowly whisk into cream. Bring to a boil, whisking constantly. Reduce heat to low; simmer, whisking constantly, 1 minute. Remove from heat; stir in 1/3 cup sugar and 1/3 cup bourbon. Set aside and let cool completely.
Preheat oven to 350 degrees.
BREAD PUDDING: In a large bowl, whisk together milk, 1/2 cup cream, eggs, sugar, vanilla, salt, and allspice. Gently stir in bread cubes, raisins, and remaining bourbon, toss to coat.
Melt butter in an 8-inch square baking dish in oven. Remove from oven, and swirl to coat bottom of dish. Pour in bread pudding mixture, distributing raisins evenly.
Bake until bread cubes are browned around edges and custard is cooked, about 30-35 minutes. Let cool completely on a wire rack. Serve warm pudding drizzled with whiskey sauce.