Heat oven to 450°. Pierce sweet potato with fork several times. Microwave on High 8 to 12 minutes or until tender. Let stand 5 minutes. Carefully peel potato. Place sweet potato in small bowl. Mash with potato masher until smooth. Stir in butter and buttermilk until smooth.
Mix flour, sugar, baking powder, baking soda, salt and 1/2 teaspoon cinnamon in large bowl. Cut in shortening, using pastry blender or crisscrossing 2 knives, until mixture looks like cornmeal. Stir in sweet potato mixture just until blended.
Place dough on lightly floured surface. Knead 10 to 15 times. Pat into 10-inch square, about 1/2-inch thick. Cut into 16 (2 1/2-inch) squares. Place squares 1-inch apart on large ungreased cookie sheet. Bake 10 to 15 minutes or until top edges are deep golden brown. Remove from cookie sheet to wire rack. Cool 5 minutes.
Mix frosting and 1/4 teaspoon cinnamon in small microwavable bowl. Microwave uncovered on High 5 to 10 seconds or until pourable. Drizzle over biscuits.