Heat oven to 350°F. Generously grease 12-cup fluted tube pan. Sprinkle with pecans, tilting pan to coat sides with some of the nuts. Set pan aside. In 1-quart microwavable bowl, place 3 ounces chocolate. Microwave on High 30 seconds; stir. If necessary, microwave in 10 second increments, stirring each time, until melted. Set aside to cool.
In large bowl beat sugar and butter with electric mixer on medium speed 1 to 2 minutes or until light and fluffy. Beat in 1/2 cup of the sour cream, the vanilla and eggs until smooth. On low speed, beat in 1 cup of the flour, the baking powder and salt until dry ingredients are moistened. Beat in remaining sour cream and flour just until blended.
Spoon 2 cups of the batter into cooled chocolate; stir until mixed. Spoon half of the remaining batter into pan. Drop half of the chocolate batter over the vanilla batter. With table knife, gently swirl. Spoon remaining vanilla batter into pan. Top with remaining chocolate batter. Gently swirl; smooth top.
Bake 38 to 45 minutes or until toothpick inserted in center comes out clean. Cool in pan 15 minutes. Invert onto serving platter.
Place 1 ounce chocolate and the oil in small microwavable bowl. Microwave on High 20 seconds; stir until smooth. Drizzle over coffee cake. Serve warm or cool.