Combine the egg yolks and the first quantity of sugar in a bowl. Whip until thick and light.
Combine the milk and the second quantity of sugar in a heavy saucepan. Bring to a simmer, stirring to dissolve the sugar.
Gradually beat the milk into the egg yolk mixture. Set over a hot-water bath and heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon. Immediately remove from the heat. Let cool until lukewarm.
Melt the chocolate and let cool slightly.
Gradually stir in the custard mixture.
Add the cocoa and beat with a whip until it is thoroughly mixed in.