Preheat oven to 375 degrees. Butter a 9-inch springform pan, and dust with granulated sugar. Whisk together flour, baking powder, and salt in a bowl. Whisk together melted butter, 3/4 cup brown sugar, the milk, and eggs in another bowl. Whisk into flour mixture.
Spread batter evenly into prepared pan. Arrange apple wedges over batter, and sprinkle with blackberries. Gently press fruit into batter. Combine remaining 2 tablespoons brown sugar and the cinnamon, and sprinkle over fruit. Dot with remaining 2 tablespoons butter. Bake until top is dark gold, apples are tender, and a cake tester inserted into center comes out clean, about 55 minutes. Let cool. Serve with whipped cream if desired.