In a bowl, cut 6 Tbs of the butter into large pats . Stir them into the flour until they disappear.
In a second bowl, combine the brown sugar, milk, mashed sweet potatoes, cinnamon and bourbon. Whisk until completely blended.
Pour the dry flour into the milk-sweet potato mixture. With a kitchen fork, stir just until most of the flour is wet. With a rubber spatula, working only around the sides of the dough mass, scrape the sides of the bowl and turn the dough a few times. Add a little more milk if more than a little dry flour remains. The dough should be moist but not sticky.
With your hands, spread the dough on a floured countertop. Fold it over like a letter into thirds, and roll it out with a rolling pin to about an inch thick.
Cut the dough with a biscuit cutter about two inches in diameter. Place the biscuits on a greased pan, spacing them about an inch apart. Put them into the oven, and lower the temperature to 425. Bake until light brown, about 12-14 minutes.
While the biscuits are in the oven, melt the remaining butter. Brush the tops of the biscuits with the butter and return to the oven for about a minute.