1. Make the lemon curd. In a heatproof bowl, stir together the lemon zest, juice and sugar. Add the butter then place over a pan of gently simmering water, stirring every so often until the butter has melted, approximately 4-6 minutes. Use a fork to whisk in the beaten egg, then switch to a balloon whisk and whisk frequently until the mixture has thickened, approximately 10 minutes. Don't be tempted to increase the heat too much as you don't want to scramble the egg. Once the mixture has thickened, remove from the heat and pass through a sieve into sterilised jars. Seal and allow to cool then store in the fridge for up to 10 days.
2. To make the trifle, cut each Swiss roll into 10 slices and place these in a shallow bowl or tray. Whisk 25g of the homemade lemon curd with the gin for kick and pour this over the sponge slices. Allow to soak for 5 minutes. Meanwhile, add the icing sugar to the cream for sweetness and whisk to soft peaks. Stir the lemon juice into the custard until well combined.
3. You're now ready to assemble! Fill the base of a trifle bowl with some of the soaked Swiss roll slices, then pour over a third of the custard and dollop on a third of the cream. Spoon over some lemon curd for zing, then repeat the process twice more until everything is used up. Chill for 4 hours or overnight before serving.
4. Just before serving, crumble over the shortbread biscuits for crunch and finish with the lemon zest.