6tablespoons (85g)unsalted buttercold and cut into cubes
Preheat the oven to 350°F. Grease a 9x13 baking dish and set aside.
In a large bowl, toss the apple slices with the brown sugar, cinnamon, and salt. Set aside.
In a separate bowl, whisk the flour, sugar, baking powder, baking soda, and salt until combined. Cut in the butter using a pastry blender or two knives until a crumby mixture forms. Add the buttermilk to the dry mixture and stir with a rubber spatula until dough forms.
Pour the apple filling into the prepared pan. Top with handfuls of slightly flattened dough. Continue until all of the dough is used up and covers the peach filling (though you will have some gaps). Bake for 45-50 minutes, or until the biscuit topping is slightly golden in color.
While any apples would work, I recommend Honeycrisp or Granny Smith.