Preheat the oven to 350 degrees F. Grease and flour a tube (not bundt) pan.
Melt the chopped chocolate bars in the top of a double boiler (or in a glass dish in short bursts in the microwave).
In a large bowl, use an electric mixer to combine the butter and sugar. Add the eggs, one at a time, beating well after each addition.
Pour the buttermilk into a small bowl and stir the baking soda into the buttermilk. Alternately mix the buttermilk and flour into the creamed mixture. Mix in the melted chocolate, chocolate syrup and vanilla.
Scrape the batter into the prepared pan.
Bake for 60 to 70 minutes, or until a toothpick inserted into the center tests clean. Remove from oven, cool, slide out of the pan, and sprinkle with powdered sugar.