Preheat the oven to 350°F. Grease a ten-inch baking dish.
In a large bowl, combine six cups of plums, one-half cup of granulated sugar, three tablespoons of cornstarch, and one tablespoon of vanilla extract.
Stir in three cups of raspberries. Transfer the filling to the prepared baking dish and bake in the preheated oven for fifteen minutes.
To prepare the topping, beat three-quarter cups of buttermilk, one egg, and one teaspoon of vanilla extract in a medium-size mixing bowl.
In a separate bowl, sift one and one-half cups of Bisquick, one-half cup of cornmeal, one-quarter cup of granulated sugar, and one tablespoon of lemon zest.
Using a pastry blender or fork, cut in six tablespoons of cold, cubed butter until crumbly.
Add the dry ingredients to the buttermilk mixture and stir until a dough forms.
Remove the filling from the oven.
Using a tablespoon, drop the dough onto the filling in evenly spaced mounds.
Sprinkle the dough with the remaining one tablespoon of granulated sugar.
Transfer to the oven and bake for thirty minutes or until a toothpick inserted into the topping comes out clean.
Allow the cobbler to cool for thirty minutes before serving.