Preheat oven to 375 degrees.
Sift together flour, soda, and salt.
Cream together butter or margerine and sugar.
Add eggs one at a time; beat thoroughly after each addition.
Add melted chocolate, vanilla, and mashed potatoes; mix well after each addition.
Grease and flour baking pans.
Bake two 8 inch round pans for 55 minutes, 9 inch tube pan for 1 hour, or 13 x 9 inch pan for 45 minutes.
Toothpick inserted in center of cake should come out clean.