Preheat your oven to 160 C (320 F).
Peel, core, and chop the apples. Add the brown sugar and cinnamon and mix to coat the apples, set aside.
Melt the butter in a small saucepan. Pour the melted butter into a 9x13 Pyrex baking dish. If using a 7x11 Pyrex baking dish, pour in about 3/4 of the butter. Move the dish around to coat the bottom, it will look like a lot, no worries.
Mix the flour and sugar in a large bowl, then whisk in the milk, then whisk in the vanilla and baking powder. Pour the batter into the baking pan and try to cover the butter, there will be a lot of butter around the sides, this is ok. DO NOT STIR.
Cover this with the prepared apples. DO NOT STIR.
Bake for 40 minutes, continue for 10 minutes at a time until the edges are browned. Remove from oven and let cool for about 10 minutes before cutting.
Serve with vanilla ice cream or whipped topping.