Cut the plums in half and remove the stone. Cut the plums into thin slices.
Place the caster sugar in a stainless steel non-stick pan and melt the sugar. After 5 minutes the sugar will become a caramel colour. At this point remove the pan from the heat and add the butter and stir well so the sugar and butter form a caramel. Add the plums and return the pan to the heat again. Allow the plums to bubble and turn the heat down. Zest the orange into the pan and allow the plums to soften for 5 minutes.
Place the plums in a bowl after 5 minutes and place into the fridge.
Lay the sponge fingers onto a shallow baking tray and cover with the rum and the coffee. Allow the sponge to soak the coffee and the rum.
Mix the caster sugar and the egg yolks together until light and fluffy. This should take about 5 minutes. The mixture should resemble hollandaise. In a separate bowl, whisk the mascarpone to loosen it. When the mascarpone has loosened, mix it together with the egg yolk mixture.
Place the sponge fingers into the bottom of a martini glass. Top the sponge fingers with the roasted plums and finish with the mascarpone mixture.
Grate some orange zest and some dark chocolate on top of the dessert and serve.
«You can now find pasteurized egg yolks in the supermarket. They save your time breaking the eggs and also have a longer shelf life.»