Preheat oven to 180°C (360°F) or 160°C (320°F) for fan-forced ovens. For the Guinness mixture - Place the butter and Guinness in a small saucepan over low heat until the butter has melted completely, but don't let the mixture simmer or boil. Remove saucepan from heat and allow the mixture to cool down for 15 minutes. This is important because we'll be mixing this mixture with the eggs so we don't want to cook the eggs. Whilst waiting for the Guinness mixture to cool down, prepare the cake tin. Grease the sides and base of a 23cm (9in) round baking tin with a loose base. Line the base with baking/parchment paper.
For the dry ingredients - Sift the flour, cocoa powder, sugar and bi-carb soda into a large bowl. Whisk the ingredients together until combined.
In a large separate bowl, place in the eggs, sour cream, vanilla extract and whisk. Then add in the Guinness mixture and whisk to combine until smooth.
Make a well in the dry ingredients, then pour in the wet ingredients, and whisk until smooth. Pour the batter into the prepared tin and tap it gently against the bench a few times. Bake in the middle of the oven for one hour or until a skewer comes out of the cake clean when inserted in the middle. Cool the cake in its tin on a cooling rack until the cake is cool to touch.
Once the cake has completely cooled down, make the cream cheese frosting. In a small bowl, whip the cream to soft peaks. In a separate bowl, place in the cream cheese, vanilla extract and whisk for 2 minutes on high speed. Sift in the icing sugar and cornstarch into the cream cheese mixture and whisk further for 3 minutes. Place two tablespoons of the cream cheese mixture into the whipped cream and whisk until combined, then fold in the remaining. Place the frosting over the cake and use a spatula to spread it over the cake. Optional - sprinkle with green decorations for St. Patrick's Day.