Prep cake batter using directions on box, swapping water for equal amount Guinness. Mix in 1 tbsp. ground cinnamon and 1 tsp. ground nutmeg. Bake as instructed. To make icing, boil 1 cup Guinness until reduced by half. In bowl, add 2 cups confectioners' sugar and whisk in reduced Guinness, 2 tbsp. at a time, to make thick icing.