Macerate 4 cups of fresh or frozen blackberries with 1 cup sugar; mash and strain. This will yield about 3 cups of syrup. Make a custard with 1 cup sugar, 1 tablespoon cornstarch, a dash of salt, 4 cups half-and-half, 2 cups whipping cream, 2 eggs well beaten and 1 teaspoon pure vanilla extract. Combine sugar, cornstarch and salt in a saucepan. Stir in half-and-half, bring to a simmer, cool, and slowly stir in beaten eggs. Bring to low heat and thicken for about 2 minutes. Remove from heat, add whipping cream, blackberry syrup and a tablespoon vanilla extract. Whisk until smooth. Refrigerate and process in the ice cream freezer. This recipe works well with berries and stone fruit.