Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper. This high temperature is crucial for creating those beautiful flaky layers.
Prepare the apple filling by combining the diced apples, brown sugar, 2 tablespoons of granulated sugar, cinnamon, nutmeg, and lemon juice in a small bowl. Set aside to allow the flavors to meld while you work on the biscuit dough.
In a large mixing bowl, whisk together the flour, baking powder, salt, and remaining 2 tablespoons of sugar until well combined.
Add the cold cubed butter to the flour mixture and use a pastry cutter or your fingertips to work it in until the mixture resembles coarse crumbs with some pea-sized pieces of butter still visible. These butter pieces create steam when baking, resulting in flaky layers.
Pour in the cold buttermilk and stir just until the dough comes together. Be careful not to overmix—a slightly shaggy dough is perfect.
Turn the dough onto a lightly floured surface and gently pat it into a rectangle about ½-inch thick. The less you handle the dough, the more tender your biscuits will be.
Spread the apple filling evenly over the dough rectangle, leaving about a ½-inch border around the edges.
Carefully roll the dough into a log, starting from the long side, sealing the seam by pinching it gently.
Cut the log into 8 equal pieces and place them on the prepared baking sheet, cut side up, about 2 inches apart.
Brush the tops with melted butter and bake for 18-20 minutes until golden brown and the centers are cooked through.
While the biscuits cool slightly, whisk together the powdered sugar and milk to create a glaze with a pourable consistency.
Drizzle the glaze over the warm biscuits and serve immediately for the best experience.