Melt chocolate and butter together over hot water. Blend in mashed potatoes, salt and vanilla. Mix well. Sift sugar. Add a small amount at a time. blending until no sugar is visible. When a spoon is no longer equal to the task of mixing, knead in the balance of the sugar with well-buttered hands. Turn out on board and continue to knead until mixture is smooth, glossy and pliable, buttering hands as necessary. No crumbs should remain. Press into buttered 8 inch square pan and cut into squares or shape into balls and top each ball with a nut. Makes 1 1/4 pounds.
*It is best to cook and mash a medium potato especially for this purpose. Omit any seasoning, butter or milk.