1. Preheat oven to 325°. Microwave chocolate baking squares in a microwave-safe bowl at HIGH 1 minute and 15 seconds or until chocolate is melted and smooth, stirring at 15-second intervals. Beat butter at medium speed with a heavy-duty electric stand mixer 2 minutes or until creamy. Gradually add granulated 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.
2. 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 (14-cup) tube pan.
3. 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 wire rack, and let cool completely (about 1 hour and 30 minutes). Sprinkle with powdered sugar, if desired. Serve with Mexican Chocolate Sauce. Garnish, if desired.
*2 (4.4-oz.) packages Mexican chocolate, chopped, may be substituted. Omit ground cinnamon.
Note: We tested with Nestle Abuelita Marqueta Mexican chocolate.