Position a rack in the center of the oven and heat to 350 degrees F.
Butter and flour a 10 or 12-cup Bundt pan and set aside.
In a small saucepan, combine the butter, cocoa powder, salt, and water and place over medium heat. Cook, stirring, just until melted and combined. Remove from the heat and set aside.
In a large bowl, whisk together the flour, sugar, and baking soda. Add half of the melted butter mixture and whisk until completely blended. The mixture will be thick. Add the remaining butter mixture and whisk until combined.
Add the eggs, one at a time, whisking until completely blended. Whisk in the sour cream (or Greek yogurt) and the vanilla extract. Whisk until smooth.
Scrape the batter into the prepared pan and bake until a toothpick inserted into the center of the cake comes out clean, 40 to 45 minutes.
Let the cake cool in the pan for 15 minutes and then invert onto a rack.
Let cool completely before glazing.
Place the chopped chocolate and corn syrup (or agave) in a medium bowl and set aside.
Combine the heavy cream and sugar in a small saucepan and put over medium heat.
Stir until the cream is hot and the sugar is dissolved.
Pour the hot cream over the chocolate and whisk until smooth.
Generously drizzle the glaze over the cooled cake, allowing it to drip down the sides.