In a bain-marie whisk the yolks and sugar until white. Put on the heat and drizzle in the marsala whilst still whisking until it becomes slightly firm. Allow to cool a little. In the meanwhile beat the ricotta with the sugar until its creamy and then mix in the chocolate. Add a little milk if it is too firm. Fold the ricotta into the zabaione and place in serving dishes. Let it rest in the fridge before serving.