Mix the egg yolks and sugar with the whisk of the hand mixer until creamy. Add mascarpone by the spoonful and stir until smooth. Stir in lemon zest. Place half of the biscuits side by side in an ovenproof dish (25 x 18 x 4 cm). Mix espresso and brandy. Soak the biscuits with half of the liquid.
Put half of the cream on the biscuits and smooth it down. Spread the rest of the biscuits on top and soak with the remaining liquid. Spread the remaining cream on the biscuits. Chill tiramisu for at least 2 hours. Dust with cocoa just before serving
waiting time approx. 2 hours