St. Patrick's Day Food: Chocolate Guinness Cake

OK, I am pretty sure that St. Patrick didn't eat this cake. But I'm also sure he'd approve.
It is, bar none, my favorite cake to make. It's easy (one pot!), and the icing is good, easy, and OPTIONAL.
(This is from Nigella Lawson's Feast: Food to Celebrate Life)

1 c. Guinness (I use extra stout, but you can use whatever kind you want)
1 stick plus 2 tbsp. unsalted butter
3/4 c. unsweetened cocoa
2 c. sugar
3/4 c. sour cream (I use light, and the cake works fine)
2 eggs
1 tbsp. vanilla extract
2 c. flour
2 1/2 tsp. baking soda

For the topping (optional): 8 oz. Philadelphia cream cheese; 1 1/4 c. confectioners' sugar; 1/2 c. heavy cream

Preheat oven to 350 degrees, and butter and line a 9" springform pan with parchment paper (the bottom only).

Pour the Guineess into a large wide saucepan or pot, and add the butter in slices. Heat until the butter's melted (about medium heat). Then whisk in cocoa and sugar. Beat the sour cream, eggs, and vanilla and then pour into the pan. Finally, whisk in flour and baking soda.

Pour the cake batter into the greased and lined pan, and bake for 45 minutes to an hour. Leave to cool completely in pan on cooling rack, as it is a quite a damp cake. (REALLY damp!)

When's the cake's cold, sit on a flat platter or cake stand and frost.

Frosting/Topping: Whip the cream cheese lightly until smooth. Sift over the confectioners' sugar and then beat them both together. Add the cream and beat again until it makes a spreadable consistency. Ice the top of the cake so that it resembles the frothy top of the famous pint.