Melt chocolate in a microwave-safe bowl at HIGH for 30-second intervals until melted (about 1 1/2 minutes total). Stir in chocolate syrup until smooth.
Beat butter at medium speed with an electric mixer until creamy. Gradually add sugar, beating at medium speed until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition.
Sift together flour, baking soda, and salt. Add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in vanilla and melted chocolate just until blended. Pour batter into a greased and floured 14-cup Bundt pan.
Bake at 325° for 1 hour and 20 minutes or until a long wooden pick inserted in center comes out clean. Cool cake in pan on a wire rack 15 minutes; remove from pan to wire rack, and let cool 30 minutes or until completely cool. Garnish, if desired.