Cream butter; gradually add brown sugar, beating well. Add eggs, one at a time, beating well after each addition. Add mashed potatoes, beating mixture well.
Combine flour, baking powder, and spices; add to creamed mixture alternately with milk, beginning and ending with flour mixture. Stir in melted chocolate and pecans.
Pour batter into a well - greased 10 - inch Bundt pan. Bake at 350° for 1 hour or until cake tests done. Cool in pan 20 minutes; remove cake from pan, and cool completely before serving.