Prepare sweet potato by slicing into ¼-inch sections. Boil until fork tender.
Drain and mash with sugar, cinnamon and vanilla. Set aside to let cool.
Sift flour into large, clean bowl.
Cut in butter and shortening until mixture appears to have pea-size lumps in it.
Make well in mixture and pour in buttermilk. With large spoon, pull from the sides of the bowl only enough to mix in wet ingredients.
Add sweet potato mixture. Mix again, only enough to distribute sweet potato mixture into dough. It should look swirled.
Turn the sticky dough out onto a floured surface, and with floured hands gently pat the dough to about ½-inch thickness.
Fold dough in half and gently pat again. Repeat 3 more times taking care not to work the dough, just carefully coaxing in layers.
Cut out biscuits with 2-inch biscuit cutter. Do not twist the cutter; push it straight down.
Transfer biscuits onto a greased cookie sheet making sure they are touching. In a small bowl, mix brown sugar, butter and pecans. Sprinkle over biscuits. Bake at 450 degrees for 20-25 minutes.