Preheat the oven to 180 °C or 160 °C fan bake. Line a 23cm / 9-inch springform tin with baking paper.
Make batter
Pour 1 cup Guinness beer into a small pot, add 1 cup coconut oil and heat until the oil is melted, remove from heat. Whisk in 75 g cocoa powder and 1 cup maple syrup. In a large bowl or stand mixer beat 150 g sour cream with 2 eggs and 1 tbsp vanilla extract. Pour the Guinness mixture into the large bowl and sift in 275 g flour and 2 ½ tsp baking soda. Whisk until just combined.
Bake cake
Pour the batter into the prepared tin and bake for 45 minutes to an hour. Leave to cool completely in the tin on a cooling rack, as it's quite a damp cake.
Ice cake
When the cake has cooled, you can sit it on a flat platter or cake stand. In a large bowl or stand mixer, lightly whip 300 g cream cheese, ¼ cup maple syrup and 2 tsp Cornflour until smooth and combined. Pour over the cake and use a spatula to spread evenly.