Place the egg yolks and superfine sugar in a large bowl and use a hand blender to whisk together until pale and thick. Add the mascarpone and whisk slowly until the mixture is pale and smooth.
To make the Guinness caramel, add the sugar and water to a saucepan, place over low heat, and stir until the sugar has dissolved. Increase the heat to high and cook for 12–14 minutes, or until the syrup is light golden brown in color. Stir in the butter and cook for another 2–3 minutes or until the mixture is a caramel consistency. Stir in the cream and Guinness, then reduce the heat and simmer for 10 minutes, or until the caramel is a thick consistency.
Dip the ladyfingers into the cold coffee and then place straight into four cocktail glasses to create the base layer. Alternatively, arrange the coffee dipped ladyfingers in a large glass bowl to make one large tiramisu.
Spoon half the mascarpone mixture over the ladyfingers, followed by a layer of the Guinness caramel and a sprinkle of cocoa, and then another layer of the remaining mascarpone mixture and Guinness caramel. Finish with a layer of cocoa on top.