Mix together the sweet potatoes, butter and milk until well blended. Stir in the flour, baking soda and sugar. Shape the dough into a ball and knead about 8 to 10 times on a well-floured board. Roll the dough out 1-inch thick and cut with a 2-inch biscuit cutter. Bake in a greased baking pan in a 400-degree oven for 15 to 20 minutes or until brown.