Instructions

• Position a rack in the center of the oven and preheat to 350°F (175°C). Butter a 10- or 12-cup Bundt pan with some of the reserved butter and dust it with flour; tap out any excess flour. Set the pan aside.
• In a small saucepan, combine the 1 cup unsalted butter, 1/3 cup cocoa powder, 1 teaspoon kosher salt, and 1 cup water. Heat over medium, stirring constantly, until the butter is melted and the mixture is smooth and combined. Remove from the heat and set aside.
• In a large bowl, whisk together 2 cups all-purpose flour, 1 3/4 cups granulated sugar, and 1 1/2 teaspoons baking soda until evenly mixed.
• Pour half of the warm butter–cocoa mixture into the dry ingredients and whisk until blended; the batter will be thick. Add the remaining butter–cocoa mixture and whisk until fully combined.
• Add the 2 large eggs to the batter one at a time, whisking well after each addition until each egg is fully incorporated.
• Whisk in 1/2 cup sour cream (or plain Greek yogurt) and 1 teaspoon pure vanilla extract until the batter is smooth and uniform.
• Scrape the batter into the prepared Bundt pan and smooth the top with a spatula. Gently tap the pan on the counter once or twice to release any large air bubbles.
• Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, about 40 to 45 minutes.
• Remove the cake from the oven and let it cool in the pan on a wire rack for 15 minutes. After 15 minutes, invert the pan onto the rack to release the cake and let the cake cool completely before glazing.
• While the cake cools, place the 4 ounces finely chopped bittersweet chocolate and 1 1/2 tablespoons corn syrup in a medium bowl and set aside.
• In a small saucepan, combine 1/2 cup heavy cream and 1 1/2 tablespoons granulated sugar. Heat over medium, stirring, until the cream is hot and the sugar is dissolved (do not let it boil). Pour the hot cream over the chocolate and corn syrup, let stand briefly, then whisk until the mixture is smooth and glossy.
• Generously drizzle the chocolate glaze over the completely cooled cake, allowing it to drip down the sides. Let the glaze set, then slice and serve.