Step 1Heat oven to 375 degrees. Lightly butter an 8-inch-square baking dish.
Step 2In a food processor bowl, pulse together the flour, 2 tablespoons sugar, baking powder, salt and orange zest. Add the cold cubed butter and pulse together just until the mixture has the texture of lightly moistened cornmeal. There may be a few pea-sized chunks of butter remaining; that's fine.
Step 3Pour over heavy cream and then pulse 4 to 6 times just to moisten the dough. Do not overmix or the dough will be tough.
Step 4In a bowl, toss together the blackberries, raspberries, one-fourth cup sugar and cornstarch to coat lightly. Add the orange juice and stir gently to moisten.
Step 5Spoon the fruit into the prepared baking dish and then use 2 spoons to scatter generous 1- to 2-tablespoon clumps of the dough over the top, covering the berries nearly completely. Bake until the topping is puffed and golden and the berries have turned jammy and fragrant, about 45 minutes.