1. Preheat the oven to 180F.
2. Grease and flour two 20 X 20 baking dish.
3. In a large mixing bowl, cream together butter and sugar, until slightly pale in color. Add in the eggs, one at a time, beat well after each addition.
4. In another bowl, place cocoa, add Guinness, bit by bit, mixing constantly until a creamy mixture is formed.
5. Gently fold in the flour to the egg - sugar mixture. Add in the cocoa mixture, little at a time, stirring well after each addition until fully incorporated. Pour into the prepared tins.
6. Bake at 180F for 30 to 32 minutes until an inserted skewer comes out clean. Allow to cook for 10 minutes. Remove from tin, allow to cool completely.
7. For icing: Mix together the butter and icing sugar together until a stiff paste is formed. Add in the Guinness, 1 tablespoon at a time, mix well after each addition.
8. Spread the icing over the cakes and decorate with pecans.