1. Microwave chocolate baking squares in a microwave-safe bowl at HIGH 1 minute and 15 seconds or until chocolate is melted and smooth, stirring every 15 seconds.
2. Beat butter at medium speed with an electric mixer 2 minutes or until creamy. Gradually add sugar, beating 5 to 7 minutes or until light and fluffy. Add eggs, 1 at a time, beating just until yellow disappears after each addition. Stir in melted chocolate, chocolate syrup, and vanilla until smooth.
3. Combine flour and next 3 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Pour batter into a greased and floured 10-inch tube pan or a 12-cup Bundt pan.
4. Bake at 325° for 1 hour and 10 minutes or until a long wooden pick inserted in center of cake comes out clean. Cool in pan on a wire rack 10 to 15 minutes; remove from pan to a wire rack, and let cool 1 hour and 30 minutes or until completely cool. Garnish, if desired.
*2 (4.4-oz.) packages Mexican chocolate, chopped, may be substituted for semisweet chocolate baking squares. Omit ground cinnamon, and proceed with recipe as directed. For testing purposes only, we used Nestlé Abuelita Mexican Chocolate.