Preheat oven to 375°F (190°C) and butter a 9x13-inch baking dish. Toss sliced apples with brown sugar, cornstarch, cinnamon, nutmeg, lemon juice, vanilla, and 2 tbsp melted butter in the dish.
Whisk flour, granulated sugar, baking powder, and salt in a medium bowl. Stir in buttermilk and 6 tbsp melted butter just until combined.
Drop spoonfuls of cobbler batter evenly over the apples and sprinkle generously with cinnamon-sugar.
Bake for 38–42 minutes until golden brown and bubbling (or cook in a paper-towel-lined Crockpot on HIGH for 2.5–3 hours). Cool 15 minutes and serve warm with vanilla ice cream!