In a large mixing bowl, combine the plums, brown sugar, brandy, grated apple, cornstarch and cinnamon. Spoon the mixture into a 9" or 10" cast iron skillet.
In a medium mixing bowl combine the flour, baking powder, salt and granulated sugar. Add the butter and using a pastry blender, cut in the butter until the mixture resembles a coarse meal. Add the cream and stir with a fork just until moistened. Gently press the mixture together using a spatula, forming a loose dough. Crumble the dough on top of the plum mixture. Sprinkle with 1 tablespoon of coarse sugar.
Bake until the top is golden brown and the filling is bubbling, about 45 minutes. Serve warm topped with ice cream if desired.