Melt the butter in a 13 x 9-inch baking dish while the oven preheats.
Combine the apples and brown sugar, tossing to coat. Spoon into the baking dish and bake for 15 minutes or until the apples begin to soften and release juices.
Meanwhile, whisk together the flour, sugar, baking powder and cinnamon in a large bowl. Add the milk, buttermilk, and vanilla and whisk until smooth.
When the apples are ready, remove the baking dish from the oven and pour the batter over the apples. Sprinkle with crumb topping.
Bake at 375°F for 1 hour to 1 hour and 10 minutes or until a wooden pick inserted in the center comes out clean. Let stand 15 minutes before serving.
Serve warm with whipped cream or vanilla ice cream.