To the cake. Heat the guinness with the butter over a low heat until it's just about all melted. Add the sugar and cocoa and whisk together until smooth. Remove from the heat and pour into a bowl. When cool enough, mix the sour cream with the eggs and add to the mix. Whisk together then add the flour and bicarbonate of soda. Combine until smooth. Pour into two lined 7" baking tins. Bake for 35 minutes at 180°C. Remove from the tins and cool the cakes on a wire rack.
Meanwhile, make the ganache. Put the chocolate in a bowl with the sugar and the salt. Pour over the hot milk, leave to stand for 1 minute then whisk together until thick and smooth. Leave to cool for 30 minutes (while the cakes cool). Spoon the ganache all over one cake, then top with the second. Press down a little.
Whip together the cream cheese, the sugar, salt and vanilla for the frosting. Fold through the whipped cream. Top your cake!