How to Make It
Preheat the oven to 375°. Butter a 10-inch cast-iron skillet and dust the bottom with the bread crumbs. In a large bowl, using an electric mixer, beat the eggs with the milk, vanilla and 1 cup of the granulated sugar. Beat in the 6 tablespoons of melted butter until incorporated, then beat in the flour until a thick batter forms.
In another large bowl, toss the 1 cup of blackberries with the sliced apples, blueberries, cinnamon and the remaining 1/4 cup of granulated sugar.
Spread one-fourth of the batter in the prepared skillet and top with the apples and berries. Using an offset spatula, spread the remaining batter over the fruit in an even layer. Scatter the remaining 8 to 12 blackberries on top and gently press them into the batter. Bake in the center of the oven for about 1 hour, until lightly golden and a toothpick inserted in the center comes out with a few moist crumbs attached. Let cool completely, then dust with confectioners' sugar. Slice into wedges and serve.
A dollop of crème fraîche or vanilla ice cream.