Cook and mash a medium potato without using any seasoning, butter or milk. Measure 1/3 cup. Melt chocolate and butter together. Blend into mashed potatoes with the salt and vanilla extract. Mix well. Sift sugar. Add a small amount at a time, blending until no sugar is visible. When unable to mix with a spoon any longer, knead in the remaining sugar with well-buttered hands.
Turn out onto a board and continue to knead until mixture is smooth, glossy and pliable, buttering hands as necessary. No crumbs should remain. Press into a buttered 8-inch square pan or shape into balls and top each ball with nutmeg.
