Grease and flour a 10-12 cup Bundt pan. Heat oven to 350°F
Melt butter in a large saucepan over medium-low heat; add cocoa, stirring until smooth. Whisk in the water and remove from heat. To the warm cocoa mixture, add the sugar, sour cream, 1 tablespoon vanilla, and eggs; whisk until smooth.
In another bowl combine the flour, soda, and salt. Add all at once to the first mixture, whisking until well blended. Pour batter into prepared pan. Bake for 40-45 minutes, or until it feels firm to the touch and has slightly pulled away from the sides of the pan. Cool in pan on a rack for 20 minutes.
Carefully loosen the cake with a knife and invert onto a large plate.
Meanwhile, prepare the glaze: Chop chocolate and put in a small bowl; set aside.
Combine remaining the heavy whipping cream, corn syrup, and 1 teaspoon vanilla extract in a small saucepan. Cook the mixture, stirring, until mixture boils. Pour over the chocolate and whisk until smooth. Let cool to room temperature then spoon over the cooled cake. If too thick, thin with a little more cream.
I give credit to this website: http://www.cdkitchen.com/recipes/recs/1931/Chocolate-Bundt-Cake-With-Dark100909.shtml.