Preheat the oven to 425 degrees F. Place a large cast-iron skillet in the oven to preheat.
Combine the self-rising flour, salt and apple pie spice in a food processor with several pulses. Drop in 1 cup cold butter cubes and pulse until pea-sized crumbs are formed in the flour. Dump this flour mixture into a large bowl. Process the sweet potato flesh in the same food processor bowl until mashed. Pour in 1 cup of the buttermilk and pulse until a smooth puree is formed. Scrape the potato mixture into the mixing bowl. Use your hands to gently pull the flour into the potato mixture, adding more buttermilk as needed to reach a moist dough. Dump the dough out onto a floured surface and press into a 1-inch-thick square. Cut out 16 square biscuits with a bench scraper, re-rolling scraps as necessary.
Melt the remaining 2 tablespoons butter in a small bowl in the microwave.
Remove the skillet from the oven and spray with nonstick cooking spray. Carefully place the biscuits in the skillet (it's okay if they touch). Brush the tops of the biscuits with the melted butter. Place the skillet back in the oven.
Bake the biscuits until fluffy and lightly golden on top, 16 to 18 minutes. Serve warm with butter and honey.