1. Heat oven to 350°F. Grease and flour 12-cup fluted tube pan.
2. Stir together chocolate bar pieces, butter and boiling water until chocolate is melted.
3. Stir together flour, sugar, cocoa, baking soda and salt in large bowl; gradually add chocolate mixture, beating until thoroughly blended. Add eggs, sour cream and vanilla; blend well. Pour batter into prepared pan.
4. Bake 55 to 60 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pan to wire rack. Cool completely. Drizzle VANILLA GLAZE over cake. 10 to 12 servings.
VANILLA GLAZE: Place 1/4 cup (1/2 stick) butter or margarine in medium microwave-safe bowl. Microwave at HIGH (100%) 30 seconds or until melted. Gradually stir in 2 cups powdered sugar, 2 to 3 tablespoons hot water and 1 teaspoon vanilla extract; beat with whisk until smooth and slightly thickened. About 1-1/4 cups glaze.