Use a 4.5cm round cutter to cut 8 rounds from the sponge. Using a small serrated knife, turn each round onto its side and carefully cut into 3 even slices. Place a slice in the base of twenty four 40ml silicone mini muffin moulds on baking trays.

Place water in a small microwave-safe bowl. Sprinkle with the gelatine and set aside for 2 minutes to soak. Microwave for 20 seconds (do not overheat). Use a fork to stir until the gelatine dissolves. Set aside for about 5 minutes to cool until just barely warm.

Meanwhile, combine the custard and limoncello in a large bowl. Use electric beaters to beat the cream in a separate bowl until firm peaks form.

Stir the gelatine mixture into the custard mixture, then fold in the whipped cream. Pour into a jug, then pour evenly over the cake slices, leaving a gap of 5mm at the top of each mould. Place in the fridge for 1 hour to set.

Place the jelly crystals in a heatproof jug. Add the boiling water and stir until the crystals are dissolved. Set aside, stirring occasionally, for about 15 minutes or until just barely warm.

Pour the jelly mixture over the top of the custard layer, filling to the brim as much as possible. Carefully place in the fridge for 4 hours or until the jelly is set. Use a small sharp knife to loosen the cakes around the edge of each mould. Unmould and arrange on a serving plate, to serve.