Cream together the butter and sugar until soft and fluffy.
Add the eggs one by one and mix well.
In a separate small bowl, mix together the flour and cocoa powder. Add about a third of the flour and cocoa powder and mix, then add a third of the buttermilk. Continue adding the flour and cocoa powder and buttermilk alternately until it is all used up.
Add the Baileys and mix well.
Prepare your slow cooker. I made this in a round 3.5 litre slow cooker, to produce a round cake. If you are using a different size or shape slow cooker you'll get different results and most likely a less deep cake. If using a 6.5 litre slow cooker double the ingredients!
Spray the slow cooker pot with cake release spray to prevent the cake sticking if it comes out of the cake case.
Place two large cake cases in the slow cooker pot. These fit well in a round 3.5 litre slow cooker. If you are using a large slow cooker you could put the cake cases in a metal or silicone cake tin.
Spoon the cake mixture into the case.
Place a tea towel under the lid (this prevents moisture from dripping onto the cake) and cook on high for approx 1.5 hours or until a cake tester or skewer comes out clean. Keep an eye on it as the time taken really depends on your slow cooker, and do watch out for burning.
Once done, remove the slow cooker pot from the slow cooker using oven gloves. Wait a few minutes for the pot to cool down, then use the edges of the cake case to remove the cake from the pot. Place it on a cooling rack to cool fully.
In the meantime, make your icing. I used a stand mixer for this, beating the butter well until very soft, then adding the icing sugar little by little on low speed until light and fluffy. Then add the Baileys and cocoa powder and beat again.
Once the cake is cool, spread the icing over the top of it.