1 cup unsalted butter, plus more for the pan
1/3 cup cocoa powder (we use Dutch processed)
1 teaspoon kosher salt
1 cup water
2 cups all-purpose Gold Medal 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 (or Greek yogurt)
1 teaspoon pure vanilla extract
ounces bittersweet chocolate, finely chopped
1 1/2 tablespoons corn syrup (or agave nectar)
1/2 cup heavy cream
1 1/2 tablespoons granulated sugar