In a large mixing bowl, combine the flour, baking powder, brown sugar and salt. Set aside.
Grate the cold butter over the flour mixture and using your hands or a biscuit cutter, rub the butter into the flour mixture until it resembles breadcrumbs. Do this quickly so the butter doesn't melt!
Place the cubed sweet potato in a microwave safe bowl and cover with a damp paper towel. Microwave for 4-5 minutes until soft. Alternatively, you can place the sweet potato in a steamer basket with an inch of water and steam for 8 minutes until bright orange and soft.
Mash the sweet potato with a fork until smooth. Mix in the buttermilk until combined.
Stir the sweet potato mixture into the flour mixture and mix until a shaggy dough forms. Lumps and shags are ok! This will help to form the flaky layers.
Turn the shaggy dough out onto a clean, floured surface and pat into a flat disk shape.
Gently roll the dough out until about 1/2 inch thick, then fold the dough over in half.
Roll the dough out and repeat the folding process again another 2 times. Be sure not to overwork the dough as this can result in hard, flat biscuits.
Fold the dough up so that it is about 1 inch thick.
Using a cookie cutter or rim of a glass, cut out 6 large biscuits (you may need to re-roll and fold the dough to get more surface area).
Place the biscuits on a parchment paper lined baking sheet and brush the tops with melted butter.
Bake the biscuits at 400F (204 C) for 15-17 minutes until golden and puffed up.
Serve warm with butter and maple syrup.