1. Preheat oven to 375°.
2. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine 2.25 ounces (about 1/2 cup) flour, apple, 1/3 cup sugar, 2 tablespoons butter, vanilla, 1/2 teaspoon salt, cinnamon, and nutmeg in a large bowl, tossing well. Spoon into a large cast-iron Dutch oven. Add 1/2 cup water.
3. Combine remaining 4.5 ounces (about 1 cup) flour, remaining 1/3 cup sugar, remaining 1/4 teaspoon salt, and baking powder in a medium bowl; cut in 1/4 cup butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add buttermilk; stir until just moist. Drop batter by tablespoonfuls over apple mixture. Bake at 375° for 1 hour or until bubbly and browned. Serve warm.
For a small gathering, halve the recipe and bake in a 12-inch cast-iron skillet.