Pre-heat your oven to 150C Fan/160C/325F/Gas Mark 3, grease and line a deep 8" tin (make sure the sides of the tin are at least 2" tall)
Melt the chocolate first. Place in a microwave safe bowl and heat for 30 seconds, stir, then heat in 10 second intervals, stirring between each one. Once fully melted, set aside to cool.
In a mixing bowl, whisk together the vegetable oil and dark brown sugar
Add the eggs and vanilla extract and whisk in
Add the Guinness bit by bit, mixing well after each addition, don't worry if it curdles a bit
Measure the self raising flour, cocoa powder salt and bicarbonate of soda out in a separate bowl and add into the mix bit by bit, mixing well after each addition
Finally, stir in the dark chocolate
Pour the mixture into the tin and bake for 30-35 minutes or until a skewer inserted in the centre comes out clean
Leave it to cool fully on a wire rack
To make the Baileys buttercream mix together the butter and the icing sugar. When it starts to come together, add the Baileys and mix until smooth. If the buttercream is too stiff then you can add some more Baileys to it!
Smooth the buttercream on top of the cake using a palette knife and decorate as you wish
Store leftovers in an airtight container in a cool place and eat within 3 days