Heat oven to 350°F. Grease a 9-inch springform pan and line the base with parchment.
Toss the chopped apples with lemon juice in a bowl and set aside.
Whisk flour, granulated sugar, baking powder, baking soda, salt, cinnamon, and nutmeg in a large bowl.
Cut in the cold butter with a pastry cutter or fingertips until sandy with pea-sized bits.
Fold in the apples to coat them evenly with the flour mixture.
Stir in buttermilk and vanilla just until no dry streaks remain; the batter will be thick.
Spread batter into the pan and smooth the top. Sprinkle evenly with coarse sugar.
Bake 50–60 minutes until golden and a toothpick inserted in the center comes out clean.
Cool 15 minutes, release the ring, and transfer to a rack. Dust with powdered sugar before slicing.