Preheat oven to 425 degrees (if you have a dark baking pan, lower by 25 degrees). Prep square or round baking pan by spraying with nonstick spray. Set aside.
In a large bowl, add flour, brown sugar, salt, baking powder, and baking soda. Grate 6 tablespoons of butter using the shredding side of a box grater and place into the flour mixture. Toss together to combine. Place into the freezer for 10 minutes.
Remove from freezer and toss in sweet potato puree until evenly combined with flour mixture. Create a well and pour in the buttermilk. Mix with a wooden spoon about 15 times around. Don't worry if it's not completely mixed.
Flour your kneading surface. Pour out biscuit mixture onto counter and work together with your hands until the mixture is no longer dry and crumbly. If the dough is too sticky, add more flour. Flour your rolling pin and begin to roll out the dough into a rectangle until it's about 1-inch thick.
Using a biscuit cutter, cut out biscuits and arrange in prepared baking pan so that the biscuits are touching. Combine remaining scraps and roll out again and cut more biscuits until all the dough is used.
Melt remaining 2 tablespoons of butter and brush over the biscuits. Bake biscuits in a preheated oven for 18-20 minutes, or until they start to brown on the edges. Remove from oven. Brush more butter over the tops. Let them cool for a few minutes.