To make the cheese filling, place 100g Haigh’s Dark Chocolate Pastilles in a medium glass bowl over a saucepan of boiling water, off the heat. Stir occasionally until chocolate has melted.
Using an electric mixer with a balloon whisk attachment, beat egg yolks, sugar and zest together until pale and mousse like. Add the mascarpone cheese beating until just combined. Slowly add melted chocolate, beating until just combined.
In a separate bowl whisk egg whites to stiff peaks. Add a large spoonful to the mascapone mixture, stirring until well combined, helping to loosen the mixture, making it easier to incorporate the remaining egg whites. Add remaining egg whites, folding into the mixture until just combined.
In a medium mixing bowl combine cooled coffee and liqueur together, stirring to combine.
To assemble tiramisu, quickly dip half the sponge fingers in coffee mixture and place across the base of a 20cm x 30cm serving dish. Top with half the chocolate mixture, spreading evenly across the sponge fingers. Repeat the process, finishing by grating the remaining 50g chocolate over the top.
Place tiramisu in refrigerator for at least 2 hours, preferably overnight, to allow flavours to develop. Serve.