In a saucepan, heat Guinness and butter over medium heat until butter is melted. Remove from heat and whisk in cocoa powder until smooth.

In a mixing bowl, combine flour, sugar, baking soda, and salt. Mix well.

In a separate bowl, whisk together eggs and sour cream until smooth. Add the Guinness mixture and stir to combine.

Add the wet ingredients to the dry ingredients and mix until just combined. Pour the batter into the prepared cake pan and bake at 350°F (175°C) for 45-50 minutes, or until a toothpick inserted into the center comes out clean.

In a small saucepan, heat heavy cream until it begins to simmer. Pour over chopped dark chocolate in a bowl and let sit for 2 minutes. Stir until smooth.

Once the cake is completely cool, pour the chocolate ganache over the top, allowing it to drip down the sides. Slice and serve this rich, moist cake!