1. To prepare horseradish butter, heat a small nonstick skillet over medium heat. Coat pan with cooking spray. Add shallots to pan; cook 1 minute. Remove from heat; cool.
2. Combine shallots, butter, parsley, and horseradish in a small bowl; blend well. Transfer butter mixture to a sheet of plastic wrap. Shape butter mixture into a 3-inch-long log, using plastic wrap to help mold. Wrap log tightly in plastic wrap, and refrigerate until firm.
3. To prepare potatoes, place potatoes and bay leaf in a large saucepan; cover with water. Bring to a boil. Cover, reduce heat, and simmer 20 minutes or until tender; drain. Discard bay leaf. Press potatoes through a ricer or food mill into a large bowl. Combine milk and broth in a microwave-safe dish. Microwave at HIGH 1 minute or until warm. Add milk mixture, sour cream, salt, and pepper to potatoes, stirring until well blended. Serve with horseradish butter.