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.
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.
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.
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.