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.
In another bowl, whisk together melted butter and 3/4 c. brown sugar, milk and eggs. Whisk into the 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 T. brown sugar and the cinnamon and sprinkle over fruit. Dot with remaining 2 T. butter.
Bake until top is dark gold, apples are tender and a cake test inserted into center comes out clean, about 55 minutes. Let cool.
Serve with whipped cream if desired. Serves 8 to 12.