Preheat oven to 180°C / 160°C fan / gas 4. Grease and line a 23cm springform tin.
Pour the Guinness into a large saucepan. Add the vegan butter (cut into chunks). Gently heat until melted.
Whisk in the cocoa powder and caster sugar until smooth.
In a separate bowl, whisk together the oat crème fraîche, Oggs egg replacement and vanilla.
Add the wet mixture into the Guinness mixture. Whisk until combined.
Sift in the flour and bicarbonate of soda. Whisk until smooth — it should be a runny batter.
Pour into the lined tin and bake for 45–60 minutes until risen and just pulling away from the sides. Cool completely in the tin — it is meant to be beautifully damp.
For the frosting, beat the vegan cream cheese until smooth. Sieve in icing sugar and cornflour and mix well.
Add the oat crème fraîche and beat until you have a light, spreadable frosting. If too loose, chill briefly before icing.
Spread the frosting over the cooled cake to mimic the creamy white head of a pint of Guinness.