1/3 cup cocoa powder (we like Penzey’s Dutch processed cocoa powder)
1 teaspoon kosher salt
1 cup water
2 cups all-purpose flour, plus more for the pan
1 3/4 cups granulated sugar
1 1/2 teaspoons baking soda
2 large eggs
1/2 cup sour cream
1 teaspoon pure vanilla extract
For the chocolate glaze:
4 ounces semi-sweet chocolate, finely chopped
1 1/2 tablespoons light corn syrup
1/2 cup heavy cream
1 1/2 tablespoons granulated sugar
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 butter/cocoa mixture and whisk until completely blended. The mixture will be very thick. Add the remaining butter/cocoa mixture and whisk until combined. Add the eggs, one at a time, whisking until completely blended. Whisk in the sour cream 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.
While the cake is cooling, make the chocolate glaze. Place the chopped chocolate and corn syrup 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.