Preheat oven to 350 degrees F. Butter a 10-inch Bundt pan and dust with cocoa powder.
In a large bowl of a stand mixer, add the butter and the sugar. Using the paddle attachment, cream together until light and fluffy and the volume has increased. Add the eggs and egg yolk, 1 at a time, beating after each addition.
In another mixing bowl, whisk together the flour, cocoa powder, the salt, and baking powder.
Add the dry ingredients alternatively with the buttermilk to the butter and egg mixture ending with the flour. Stir in the vanilla and pour the batter into the prepared Bundt pan. Bake for 1 hour and 10 minutes or until a toothpick inserted in the cake comes out clean. Let cool on a wire rack before turning out onto a serving plate and slicing.
Add the chocolate to a medium-sized bowl. In a small saucepan over low heat, add the cream and bring to a low simmer. Pour the cream over the chocolate and add the corn syrup. Once the chocolate melts, whisk together to combine. Drizzle over the cake and serve.