Weigh or lightly spoon flour into dry measuring cups; level with a knife.
Combine 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.
Combine remaining 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.