Preheat oven to 375 F
In a mixing bowl, combine plums, blackberries, brown sugar, vanilla, cornstarch and cinnamon.
Pour mixture into a 9" or 10" iron skillet.
In a stand mixer or with a hand mixer, combine flour, baking powder, salt, and sugar.
Add the butter and blend until the mixture resembles a coarse meal.
Pour in heavy whipping cream and stir until just blended
Crumble mixture on top of plum and blackberry mixture
Bake for 45 minutes
Serve warm with vanilla ice cream or whipped cream