In a large saucepan, cook potatoes in boiling water to cover, for 15 minutes, or until tender. Drain, and let potatoes cool to touch; peel and mash.

In a small bowl, combine 1 cup mashed sweet potatoes and 2 tablespoons butter, stirring until butter is melted; set aside.

Preheat oven to 425 degrees.

In a large bowl, combine flour, sugar, baking powder, and salt. Cut in 4 tablespoons butter with a pastry blender until mixture is crumbly. Add milk and mashed sweet potatoes, stirring with a fork until dry ingredients are moistened. (Dough will be sticky).

Turn dough out onto a heavily floured surface, and knead lightly 4 or 5 times.

Roll dough to 3/4-inch thickness, cut with a 2-inch biscuit cutter, and place on a lightly greased baking sheet.

Bake for 12 to 14 minutes.