To make the cake, preheat the oven to 325 degrees F. Butter a 12-cup or 14-cup Bundt pan. In a large bowl, combine the flour, cocoa powder, baking soda and salt. In a glass measuring cup, mix together milk and sour cream.
In an electric mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until light and fluffy, about 5 minutes. Add eggs, one at a time, beating well after each addition. Add vanilla and reduce speed to low. Add flour mixture in two batches, alternate with the milk mixture, beginning and ending with the flour mixture. Mix until just combined.
Pour the batter in the prepared pan and smooth top with an offset spatula. Bake until toothpick can be inserted into the center comes out clean, about 50-55 minutes. Transfer pan to a wire rack to cool completely.
To make the glaze, place the chopped chocolate in a heatproof bowl; set aside. Heat the cream in a small saucepan over medium heat until it just begins to bowl. Remove from heat and pour over chocolate. Let stand one minute, then add butter and stir until smooth. Let stand, stirring occasionally until slightly thickened. Pour glaze over cake.