Pour 1/2 cup milk into a small saucepan; bring to a simmer over medium heat. Place cornstarch in a small bowl; gradually add water, stirring with a wire whisk until blended. Add to simmering milk; bring to a boil, stirring constantly. Cook 1 minute, stirring constantly.
Combine cornstarch mixture, remaining 1 cup milk, and vanilla in a large bowl; freeze mixture 1 hour or until a 1/8-inch-thick layer of ice forms on the surface. Beat mixture at high speed of an electric mixer 5 minutes; gradually add sifted powdered sugar, beating until soft peaks form. Cover and chill.
Combine coffee, whiskey, and sugar; stir well. Pour into mugs; spoon topping onto coffee. Serve immediately. Cover and store remaining topping in refrigerator.