Place the chocolate in a double boiler, and melt over simmering water until smooth. Alternatively, place chocolate in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until chocolate is melted and smooth. Remove from heat if using double boiler and set aside.
Place egg whites in the bowl of a stand mixer, and beat with whisk attachment until soft peaks form. Slowly whisk in 1/2 cup plus 2 tablespoons of the sugar. Continue to beat until the whites form soft peaks when mixer is stopped and beaters are lifted, about 1 to 2 minutes.
Stir the egg yolks into the melted chocolate until combined, and then stir in 1/3 cup cream, Guinness and espresso until evenly combined. Gently fold in the egg whites with a rubber spatula until there are only trace streaks of white. Divide mousse among 6 mugs or pudding cups.
In a clean bowl, beat the remaining 1 cup cream with the remaining 3 tablespoons of sugar and the whiskey until soft peaks form. Spoon on top of the mousse and refrigerate for 1 hour.