Chocolate Buttermilk Cake - Slow Cooker

1. Grease and line a 9" (18cm) deep cake tin with greaseproof paper. Place an upturned saucer in the slow cooker cooking dish and pour in 1" (2.5cm) of cold water. Turn the slow cooker to high. 2. Cream together the butter, sugar and vanilla essence until light and fluffy, then beat in the egg yolks one at a time. 3. Sift the flour and cocoa powder in to the egg mixture and fold together, adding the buttermilk. 4. Whisk the egg whites until soft peaks form. Fold carefully into the cocoa mixture in two matches. Spoon the mixture into the prepared tin and cover with a piece of lightly oiled tin foil. 5. Place the tin in the cooking dish and pour around enough boiling water to come halfway up the side of the tin. Cover with the lid and cook in the slow cooker for 4 to 6 hours, until firm. 6. Cool slightly, then remove from the slow cooker and leave to cool for an additional 30 minutes. Turn out onto a wire rack and only turn it out when it is completely cold. 7. To prepare the icing, place the butter and chocolate together in a heatproof bowl and set over a saucepan of simmering water. Stir until smooth and melted, then leave at room temperature until it is thick enough to spread. 8. Split the cake horizontally in half and use some of the chocolate mixture to sandwich the two halves together. Spread the top of the cake with the remaining chocolate mixture. 9. Whip the cream until soft peaks form and spread this around the side of the cake. Chill in the fridge until required and serve in slices.