Break the dark chocolate into bits and melt in bath in a heat-resistant bowl over simmering water. Put it aside.
Whisk the egg yolks and sugar until light and creamy.
Heat the cream till simmering point, then take it off the stove and pour it slowly into the egg mixture, stirring constantly with a mini hand whisk. Pour the mixture back into the pan, add the chocolate sauce and the cocoa, and cook over low heat until you get a thick cream, stirring continuously. Don’t allow the mixture boil or it will curdle.
When the cream coats the back of a wooden spoon, the custard is ready. Pour it into a jug and place it in a bowl of cold water.
When the custard is completely cooled, pour it into a suitable container, cover and place it in the coolest part of the freezer. Using an electric mixer, whisk the mixture every hour or so- at least 3-4 times. If you have and ice cream maker, churn the mixture according to the manufacturer’s instructions, until it’s frozen.