Preheat the oven to 350°F. Generously butter a 10- to 12-cup Bundt pan, then dust it lightly with flour, shaking out any excess.
Make the chocolate cake batter. In a small saucepan, combine 1 cup unsalted butter, 1/3 cup cocoa powder, 1 teaspoon salt, and 1 cup water. Heat over medium heat, stirring occasionally, just until the butter has melted and the mixture is smooth. Remove from the heat.
In a large bowl, whisk together 2 cups all-purpose flour, 1 3/4 cups granulated sugar, and 1 1/2 teaspoons baking soda.
Pour half of the warm butter mixture into the dry ingredients and whisk until combined. Add the remaining butter mixture and whisk until smooth.
Add 2 large eggs, one at a time, whisking well after each addition.
Add 1/2 cup sour cream and 1 teaspoon vanilla extract. Whisk until the batter is smooth and evenly combined. Do not overmix.
Pour the batter into the prepared Bundt pan and smooth the top.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the cake cool in the pan for 15 minutes. Carefully invert it onto a wire rack and allow it to cool completely before glazing.
Make the chocolate glaze. Finely chop 4 ounces chocolate. In a small saucepan, combine 1/2 cup heavy cream and 1 1/2 tablespoons granulated sugar. Heat over medium heat, stirring, until the cream is hot and the sugar has dissolved. Do not bring it to a boil.
Remove the cream mixture from the heat. Add the chopped chocolate and whisk until completely melted and smooth.
Drizzle the chocolate glaze generously over the completely cooled cake, allowing it to drip down the sides.
Let the glaze set slightly, then slice and serve.