Here is the belated (but worth it, IMO!) Guinness Malt Cake recipe. Personally, I don't think the icing is essential.
Guinness Malt Cake
250ml Guinness or other stout beer
250g unsalted butter
1 tbs molasses or black treacle
200g dark brown sugar or molasses sugar
70g cocoa powder
2 tbs powdered malt (e.g. Horlicks/Ovaltine)
2 eggs
150g natural yoghurt
280g plain flour
2 tsp bicarbonate of soda
200g caster sugar
pinch of sea salt
For the frosting:
100ml Guinness or other stout beer
60g unsalted butter, softened
120g cream cheese, softened
½ tsp vanilla extract
300g icing sugar, sifted
Preheat oven to 160C/325F/GM3. Butter a 900g/2lb loaf tin and line it with baking paper.
Place Guinness, butter, molasses and brown sugar in a small saucepan and melt over a medium heat.
Whisk in the coca and malt, then take off the heat.
In a large bowl, whisk together the eggs and yoghurt, then add the stout mixture.
Sift together the remaining dry ingredients into the bowl and whisk all together to combine. Pour into the prepared tin and bake in the oven for about 1 hour 15mins, or until a skewer inserted comes out clean. Cool the cake completely in the tin.
While the cake is cooling, make the icing. Put the Guinness in a small pan and bring to the boil. Boil for about 10-15 minutes, or until the beer has reduced by half its volume. Pour into a container and put in the fridge to cool down [think this goes against food hygiene recommendations, but anyway...]
In a mixing bowl, beat the soft butter until creamy and light. Add the cream cheese and beat until smooth. Add the vanilla and the sifted icing sugar and beat well. Now add the cooled, reduced Guinness and beat until creamy and light. Turn the cake out of its tin and spread the icing on top.