Preheat oven to 450°F.
Cook apples in 2 tbsp butter with brown sugar, cinnamon, and nutmeg for 4–6 minutes, until slightly softened and syrupy.
Remove from heat and stir in vanilla.
Melt 1 stick butter in a 9x9 baking dish.
Whisk flour, baking powder, salt, sugar, and cinnamon.
Stir in buttermilk, 2 tbsp melted butter, and vanilla, then fold in apples.
Spread batter over melted butter, score into 9 biscuits, and top generously with turbinado sugar.
Place dish on a lined sheet pan and bake 30–35 minutes, loosely tenting with foil around 25-27 minutes if well browned.
Serve warm with butter and maple syrup, if desired
