Melt chocolate and butter together. Blend into mashed potato with the salt and vanilla extract. Mix well.
Sift sugar. Add a small amount of sugar to potato mixture, a little at a time, blending until no sugar is visible. When a spoon is no longer able to mix , knead in the balance of sugar with well-buttered hands. Turn out on a board and continue to knead until mixture is smooth, pliable and glossy, buttering hands as necessary. No crumbs should remain. Press into a buttered 8-inch square pan.
