Heat oven to 350°F. Generously grease 12-cup fluted tube cake pan or 10-inch tube pan. Break 4 oz. chocolate into pieces; place in large bowl. Add potato flakes; pour boiling water over flakes and chocolate. Let stand 5 minutes or until potato flakes are softened and chocolate is melted; stir to combine.
Lightly spoon flour into measuring cup; level off. Add flour and all remaining cake ingredients except pecans to potato flake mixture; blend at low speed until moistened. Beat 3 minutes at medium speed. Stir in pecans. Pour batter into greased pan.
Bake at 350°F. for 45 to 60 minutes or until toothpick inserted near center comes out clean. Cool upright in pan for 30 minutes. Invert onto serving plate. Cool 1 hour or until completely cooled.
In small saucepan over low heat, melt 4 oz. chocolate with 3 tablespoons water and 1tablespoon margarine. Remove from heat. Add powdered sugar and 1/2 teaspoon vanilla; beat until smooth. Stir in additional water, a few drops at a time, if needed for desired glaze consistency. Immediately spoon glaze over cooled cake, allowing some to run down sides.