Preheat oven to 375 degrees F (190 degrees C). Grease and flour a 9x13 inch pan. Sift together the flour, baking soda and salt. Set aside.

In a large bowl, cream together the margarine and sugar until light and fluffy. Beat in the eggs one at a time. Stir in the melted chocolate, vanilla and mashed potatoes. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated.

Pour batter into prepared pan. Bake in the preheated oven for 45 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.