Preheat oven to 325 degrees F. Grease and flour a tube or bundt pan; set aside.
Place the chocolate in a microwave safe bowl and heat on high for 1 minute and 30 seconds, stirring every 15 seconds until the chocolate is melted and smooth; set aside. In a large mixing bowl, beat the butter on medium high speed until creamy, about 2 minutes. Gradually add the sugar, beating for 5 to 7 minutes, until light and fluffy. Add eggs, 1 at a time, fully mixing into the batter after each addition. Stir in the melted chocolate, chocolate syrup, and vanilla extract until incorporated.
In a separate medium bowl, whisk together the flour, cinnamon, baking soda and salt. With the mixer on low-speed, alternately add the flour then the buttermilk to the butter/sugar mixture in 3 to 4 stages, starting and ending with the flour. Blend well after each addition. Pour the batter into the prepared pan.
Bake at 325 for 1 hour and 10 minutes, or until a long wooden pick inserted in the center of the cake comes out clean. Cool in the pan on a wire rack for 15 minutes, then invert the cake out from the pan onto the wire rack and allow to cool completely (about 2 hours). Once completely cooled, sprinkle with powdered sugar and cut into 1-inch slices.
Source: Adapted from a Southern Living postcard