Preheat the oven to 180°C (350°F) and grease a 9-inch springform cake pan.
In a saucepan over medium heat, combine Guinness stout and butter; heat until the butter is melted.
Remove the saucepan from heat, whisk in cocoa powder and sugar until well combined.
In a separate bowl, beat together sour cream, eggs, and vanilla extract until smooth.
Pour the sour cream mixture into the Guinness mixture and mix well.
Gradually add the sifted flour and bicarbonate of soda, folding gently to combine.
Pour the batter into the prepared cake pan and smooth the surface.
Bake in the preheated oven for 45 minutes, or until a skewer inserted into the center comes out clean.
Cool the cake in the pan for at least 10 minutes before transferring to a wire rack to cool completely. Decorate with chocolate.