In a saucepan, heat the sugar and water until the sugar is completely dissolved.
Remove from heat and add the Champagne; stir in the creme de cassis.
Pour mixture into 2-quart container (sides should be about 2 inches high) and place in the freezer.
Begin checking it after about 1 hour; once it begins to freeze around the edges, take a fork and stir the mixture, breaking up the frozen parts near the edges into smaller chunks and raking them toward the center.
Return dish to freezer, then check mixture every 30 minutes afterward, stirring each time and breaking up any large chunks into small pieces with a fork, until you have beautiful, fine crystals.
Because the granita may melt rapidly, suggest spooning into chilled goblets and letting rest in the freezer until ready to serve.