Preheat the oven to 350°F. and place a rack in the middle of the oven. Grease and lightly flour a 9-inch cake pan (or use an 8-inch pan if you'd like a slightly taller cake.)
PREPARE THE CAKE:
In a small saucepan, combine the cocoa powder and the Guinness. Heat over medium-low heat until smooth and warmed. Set aside to cool.
In a medium bowl, sift together the flour, baking soda, baking powder and salt. In a large bowl use an electric mixer to beat the butter until creamy. Gradually add the sugar and beat until pale yellow in color. Beat in the eggs, one at a time, and the the vanilla.
Stir the buttermilk into the cooled cocoa and stout mixture.
With the mixer on low, slowly add ⅓ of the buttermilk mixture into the creamed butter until incorporated. Add the flour mixture in three parts, alternating with the remaining two parts of the buttermilk and ending with the flour. The batter will look grainy or appear to be breaking up.
Pour the batter into the prepared cake pan and bake 25 to 35 minutes or until the cake pulls away from the sides of the pan and a toothpick inserted into the middle comes out clean. Remove to a wire rack with parchment or waxed paper beneath it. Allow the cake to cool in the pan for 10 minutes, then slide a knife around the edge of the pan and invert the pan to release the cake, bottom side up, onto a wire rack. When cool, use a fork to poke holes generously into bottom side of cake.
ADD THE SAUCE:
In a small saucepan, mix the sauce ingredients and heat over low heat until smooth. Allow to cool. Spoon ¾ of the sauce over the bottom of the cake, allowing the sauce to seep into the cake. Invert the cake right-side-up onto a serving platter. Poke holes in the top of the cake and spoon the remainder of the sauce on top of the cake.
ADD THE GLAZE:
In a small saucepan, bring the cream to a simmer. Turn off the heat and stir in the chocolate until the sauce is smooth and creamy. Pour onto the finished cake, smooth the ganache on top and along the sides of the cake. The ganache will pool at the bottom of the cake and can be removed with a knife, if desired.
Notes
*If desired, spoon extra ganache into a zip baggie, snip the corner and squeeze on top of the cake in zigzag lines or a decorative pattern. If the ganache is too thin, allow it to thicken in the bag until thicker.
*Reserve any extra ganache in the refrigerator to spoon over ice cream or form into truffles.