Allow butter and eggs to stand at room temperature for 30 minutes. Meanwhile, grease and lightly flour a 10-inch fluted tube pan. In a medium bowl stir together flour, baking powder, baking soda, and salt. Set aside.
Preheat oven to 350 degrees F. In a large mixing bowl beat butter with an electric mixer on low to medium speed about 30 seconds. Add sugar and vanilla; beat until fluffy. Add eggs 1 at a time, beating on low to medium speed 1 minute after each addition and scraping bowl frequently. Alternately add flour mixture and buttermilk to butter mixture, beating on low speed after each addition just until combined. Reserve 2 cups batter. Pour remaining batter into the prepared pan.
In a small mixing bowl combine chocolate-flavored syrup and reserved 2 cups batter. Beat on low speed until well combined. Pour chocolate batter over vanilla batter in pan. Do not mix.
Bake about 50 minutes or until wooden toothpick inserted near center comes out clean. Cool 15 minutes on wire rack. Remove cake from pan; cool thoroughly on wire rack. Drizzle cake with Semisweet Chocolate Icing.
Makes 12 servings.
Semisweet Chocolate Icing
In a small saucepan heat semisweet chocolate pieces, butter, corn syrup, and vanilla over low heat, stirring until chocolate melts and mixture is smooth. Use immediately.