Sauce for fruit or ice cream. Yields about 1.5 cups for standard serving size. Blueberries may be used instead of strawberries. Cook berries in sugar (more if fresh) just until hot. Add lemon juice, grated lemon rind and simmer 5 minutes. Force through a sieve. Mix together cornstarch and cognac till smooth; Add berry mixture and cook over low heat, stirring constantly until it is clear and thickened. Sauce may be served hot or cold. Serve over fruit or ice cream.