Here it is, just in case:
225g/ 8oz unsalted butter, softened
350g / 12 oz soft dark brown sugar
4 large eggs, beaten
225 g/ 8oz plan flour
1/2 tsp baking powder
2 tsp bicarb
400ml guinness (allow head to settle before using)
100g/ 3 1/2 oz cocoa powder
150g dark chocolate (60-70% cocoa solids, grated)
Oven 180C/350F/GM4
Butter & line 23cm/9 in deep springform tin
Cream butter & sugar. Gradually add eggs. Sift flour, baking powder & bicarb together. Mix Guinness & cocoa in jug (you will need to persevere with this) Add grated chocolate to Guinness mix.
Add flour & Guinness mix alternately to egg mix, stirring between each addition, until completely mixed. Resulting mix will be quite soft.
Spoon in to tin. Bake for 1-1 1/4 hours until skewer comes out clean. You may need to cover the top with foil or greaseproof paper after a while (check after 45 mins, I'd say) to stop top from burning.
Leave to stand for 10 mins before turning out.
I reckon you could just go for whipped cream on top, or even melted white chocolate, if you want the Guinness look. Otherwise, chocolate glaze:
100g dark choc
40g unsalted butter (cubed)
3 tbs water
75 g icing sugar
Melt chocolate in bowl over hot water. Leaving bowl over water, sift in icing sugar, stir well, then add butter and stir well. Remove from heat, add water, 1 tbs at a time. Use while still warm, but not too hot.