Sauce for fruit or ice cream. Yields about 1 1/2 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.