Using the whisk on a hand mixer, beat the sugar, egg yolks and water for approx. 5 mins. until the mixture is light and fluffy. Beat the egg whites with the salt until stiff, add the sugar, continue beating until the egg whites turn glossy. Combine the flour and cocoa powder, sieve into the mixture in layers along with the egg whites and carefully fold in using a rubber spatula. Transfer the cake batter to the prepared tin.
Approx. 30 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove from the tin and leave to cool completely on a rack.
Mix the espresso and Baileys. Cut the cake in half crosswise, drizzle the mixture over both halves.
Whisk together the mascarpone, Baileys and icing sugar. Beat the cream until stiff, fold into the mascarpone mixture, spread 1/3 over the base of the cake, cover with the top half of the cake, spread the remainder of the mascarpone cream around the sides. Cover the cake and leave to set in the fridge for approx. 1 hr. Dust with cocoa before serving.