Preparation : Preheat oven to 350 degrees. Melt chocolate with the vanilla; cool slightly. Sift together the flour, baking powder, soda, salt and cinnamon. In a large bowl, cream the butter and sugar and beat in the eggs one by one. Add the chocolate and mashed potatoes. Beat in the dry ingredients alternately with the buttermilk. Pour into a greased 13x9 inch pan and bake in a preheated 350 degree oven for approximately 40 minutes (or bake in 2 round 9 inch pans for 30-35 minutes). Cool before frosting. (Makes a 13x9 inch cake or two round 9 inch layers.) Melt chocolate and vanilla. Let cool slightly. Cream the sugar, salt and butter. Add just enough hot milk to make a spreadable consistency. Beat in the melted chocolate. Spread on the cake and sprinkle with the chopped nuts.