Halve the plums and discard the stones. The Illawarra plums have very small stones so just put them in whole. Put the plums into a small saucepan with the orange juice, zest and sugar, and bring to the boil. Reduce the heat and simmer gently until the plums are falling apart. Add the cherries right at the end to keep their texture. Leave to cool.
Cut sponge cake into 2 cm slices. Select a serving bowl and layer the cake slices in the base and a little way up the sides.
Boil the water in a kettle, pour into a bowl and mix the hot water with the lemon myrtle powder until dissolved.
Stir in the limoncello (add more or less limoncello directly to the sponge depending on how soggy you like the cake) and pour the mixture over the cake. Push the cake down to soak up the liquid.
Whip the cream with the sifted icing sugar (and wattleseed extract, if using) until thick. Swirl the custard through the cream mixture. To assemble the trifle, spoon the cooled plum mixture on to the cake, then the custard mixture. Refrigerate 30 mins before serving.
Note: This recipe can be made up to one full day before serving.
This recipe is from Warndu Mai (Good Food) by Rebecca Sullivan & Damien Coulthard published by Hachette Australia, Hardback RRP $45. Our thanks to them and SBS.