Preheat oven to 400°F. Grease a 2-quart baking dish or a 9-10 inch cast iron skillet.
PREPARE THE FILLING:
In the prepared baking dish or in a separate bowl, toss together the apple slices with the sugar, lemon juice, cinnamon, nutmeg, allspice, and salt. Set aside.
PREPARE THE TOPPING:
In a large bowl, whisk together self-rising flour, sugar, salt, cinnamon, and nutmeg. Add the grated butter and toss gently to combine, so that all of the butter is coated in the flour mixture. Stir in 1 cup of cold buttermilk just until combined. The dough should just come together, without any dry pockets of flour. Add a splash of extra buttermilk, if necessary.
Drop the dough onto the apple mixture in 6 equal portions, about ¾-inch - 1-inch thick.
Brush the tops of the biscuits with the remaining 2 tablespoons of buttermilk; sprinkle with coarse sugar.
BAKE THE COBBLER:
Bake, uncovered, for 35-40 minutes, until the fruit filling is bubbling and the biscuits are golden brown and cooked through. Tent the dish loosely with foil towards the end of the baking time if the top starts to get too dark before the cobbler is done.
Serve warm or at room temperature with vanilla ice cream.
Notes
Allow the apple filling to sit and rest for 10-20 minutes while you make the biscuit topping. The sugar will release some of the fruit's juices, which you want to include in the skillet when you prepare the dessert.
The butter and buttermilk need to be as cold as possible for flaky and tender biscuits. You can freeze the cubed butter for about 10-20 minutes before using it, or keep it in the refrigerator. I also keep the buttermilk in the refrigerator right up until it’s time to add it to the dough.
The dollops of biscuit dough on top of the fruit do not need to be perfect. We're going for a "rustic" look! I like to portion out about 6 equal pieces of dough with a knife or by hand, and then use my hands to gently press the dough into rounds that are about 1-inch thick. There's no need to roll the dough or pull out a biscuit cutter, although you certainly can do so if you like.
The cobbler is done when the filling is bubbly and the biscuits are cooked through. Make sure that the biscuits are totally cooked through by sticking a toothpick in the center of one. If it comes out clean, you'll know they're fully set.
If the tops of the biscuits start to get too dark before they are completely cooked through, you can tent the dish loosely with foil during the final few minutes of baking.