Using both butter and avocado oil gives you the best of both worlds, buttery flavour and a sponge that stays soft for days. | Keep the icing thick rather than runny for that classic school cake look. | Let the cake cool completely before icing or the icing will melt and lose definition. | The cake is often even better the next day once the jam and icing have settled into the sponge. | Store covered in the fridge for up to 3 days. Bring to room temperature before serving or warm gently in the microwave and serve with extra custard. Frozen slices also thaw brilliantly for an easy dessert.