Mix the sugar and egg yolks in a bowl with the bamix® whisk until the mixture is light in colour. Grate a little of the lemon and orange zest and stir into the mixture together with the mascarpone. Using the bamix® beater, beat the egg whites with the salt until stiff. Add 1 tbsp sugar and continue beating briefly until the foam is glossy. Carefully fold into the mascarpone mixture, cover and refrigerate. Stir the coffee and 1 tbsp sugar until the sugar has dissolved, then add the coffee liqueur and allow to cool. Cover the bottom of the tray with half of the sponge fingers. Drizzle half of the coffee liquid over the sponge cake. Then spread half of the mascarpone crème on top, dip the remaining sponge fingers in the liquid and place on top. Cover with the rest of the crème, smooth it out, cover and refrigerate for at least 2 hours. Dust the Tiramisu with cocoa powder before serving.