Add the guinness to a saucepan and bring to a simmer over a medium heat. Keep a close eye on the pan as it may bubble over the sides of the pan once heated initially. Continue to simmer for about 20-30 minutes until the mixture has reduced to 200ml. Once reduced, transfer the liquid to a heat resistant jar and place it in a cold water bath to bring the guinness reduction to room temperature.
Meanwhile, to make the crust add the rolled oats, cacao powder and salt to a food processor and blitz to a fine crumb consistency. Add the dates and butter/coconut oil and continue to process until the ingredients stick together to form a dough. If the ingredients do not stick together at this stage, you can add 1-2 tablespoons of water and blend to form a dough.
Line the bottom and sides of a 7.5 or 8" cake tin (with removable base) with some parchment paper. Press the dough into the base of the cake tin, and smooth it out with a spatula or flat bottomed glass. Set aside in the fridge.
For the filling, melt the chocolate over a bain-marie (double boiler). Add the guinness reduction, melted chocolate, silken tofu, vegan cream cheese, cacao powder and lemon juice to a high speed blender and blend until smooth and creamy.
Transfer the filling into the cake tin using a spatula to smooth it out. To get the swirl effect on top, place the cake tin on top of a turntable, and use your spatula to make a swirl print on the surface of the cake whilst spinning the turntable. Place in the fridge to set for 3-4 hours (or overnight if preferred).
Once set, remove the cake from the tin. You can use a blow torch to gently heat the sides of the pan to make it easier for the cake to slide out of the tin.
Garnish with some grated chocolate.
Store in an airtight container in the fridge and consume within a 5-6 days.