Preheat oven to 180 C / Gas 4. Line 2 muffin or bun trays with paper cases.
Bring Guinness® and butter to the boil; set aside until butter has melted, stirring occasionally. Stir in cocoa powder until smooth.
Whisk together flour, sugar, bicarb and salt in a bowl until thoroughly combined.
In a different bowl, beat eggs and soured cream together until well combined.
Slowly whisk or beat in the Guinness mixture, then the flour mixture; beat until the cake mixture is smooth. Spoon evenly into 24 bun cases, filling each one about 2/3 full.
Bake in the preheated oven until a tskewer inserted into the centre of a cupcake comes out clean, about 17 or 18 minutes. Cool completely on a wire rack.
To make the icing:
In a heavy-bottomed small saucepan, beat egg with fork. Add milk and sugar, whisking thoroughly. Put on medium-low heat and cook, whisking constantly to prevent burning. Bring to the simmer and cook until mixture thickens slightly (it still will be somewhat runny consistency). Remove from heat and let cool completely (you can cover it and cool in the fridge at least for 30 minutes).
Whip softened butter using electric mixer until fluffy, at least for 5 minutes. Gradually add the cooled sauce, 1 to 2 tablespoons at a time, beating after each addition. Add Baileys® and cocoa powder and whip until the icing becomes light and fluffy. Total time for whipping frosting should take about 15 minutes. Put bowl with frosting in the fridge for 1 hour to get a good piping consistency.
Use a piping pag and nozzle to decorate cupcakes.
I used half salted butter, half unsalted and liked it that way. You can also add a pinch of salt to the icing, if you like. I think salt adds a nice twist, but tastes are very personal.
Made these twice, first time the cupcakes pulled away from the wrappers and shrank but were still delicious. Second time I upped the baking soda to a full tsp and subbed 50g plain flour for self-raising and they were just as good but didn't shrink. Made the full 24 cupcakes with the perfect amount of frosting.
Note: Would make the egg/milk/sugar mix for the frosting before making the cakes so it has enough time to cool. - 20 Oct 2015
I made these for a St Patricks day theme day at work and they took a while and (yes you need to have some cooking skills) but my god they were fantastic and the icing was out of this world, it was the best icing I have ever made ( and I have been a cook for over 20 years )I am going to experiment to adapt it using other flavours and see what happens. the cupcakes were so moist ( and the mixture is quite runny) that you think you have measured out wrong but they cook really well. - 20 Mar 2015