8 ounces (2 sticks) unsalted butter, plus more for pan
2 1/4 cups all-purpose flour
3/4 cup unsweetened cocoa powder
1 teaspoon baking soda
1 teaspoon salt
1/2 cup milk
1/2 cup sour cream (4 ounces)
1 1/2 cups sugar
4 large eggs
1 teaspoon pure vanilla extract
FOR THE GLAZE
3 ounces bittersweet chocolate, chopped
1/2 cup heavy cream
2 tablespoons unsalted butter
Make the cake: Preheat oven to 325 degrees. Butter a 14-cup Bundt pan. In a large bowl, whisk together flour, cocoa, baking soda, and salt. Mix milk and sour cream in a glass measuring cup.
With an electric mixer on medium-high speed, beat butter and sugar until pale and fluffy, 3 to 5 minutes. Add eggs, 1 at a time, beating well after each addition; add vanilla. Reduce speed to low; add flour mixture in 2 batches, alternating with the milk mixture and beginning and ending with the flour; beat until just combined.
Transfer batter to prepared pan; smooth top with an offset spatula. Bake until a cake tester comes out clean, about 55 minutes. Transfer pan to a wire rack to cool completely.
Make the glaze: Place chocolate in a heatproof bowl. Bring cream to a simmer in a small saucepan; pour over chocolate. Let stand 2 minutes. Add butter, and mix until smooth. Let stand, stirring occasionally, until slightly thickened. Pour glaze over cake.