In a large bowl, mix grapes with sugar, creme de cassis and cornstarch until the fruit is evenly coated. Transfer to a 2-qt. baking dish.
In a medium bowl, whisk the flour, sugar, baking powder and salt. Add 3/4 cup plus 3 tbsp. cream and mix just until the dough comes together.
Using a spoon, dollop the dough in craggy mounds, spacing evenly on top of the grape mixture. Dab with cream, then sprinkle them with raw sugar. Bake at 400 degrees until the filling bubbles and a toothpick inserted into the biscuits comes out clean, tenting with foil if the biscuits are browning too quickly, 35 to 40 minutes.