Place the egg yolks in a heatproof bowl.
Add the sugar and beat the egg yolks and sugar until the mixture becomes pale and thick. Then add the cocoa powder.
In a saucepan, combine the milk, cream, and vanilla bean. Bring to a boil.
Gradually add the hot milk mixture to the egg mixture, stirring gently.
Place the bowl over a saucepan of hot water and stir constantly over low heat until the custard coats the back of a spoon, about 20 minutes.
Strain the custard through a fine-mesh sieve to ensure there are no lumps.
Melt the dark chocolate in a double boiler and stir it into the vanilla custard, stirring constantly until the mixture has cooled.
Refrigerate the chocolate mixture for 12 hours.
Freeze the ice cream maker bowl for 12 hours.
Pour the chocolate mixture into the ice cream maker bowl and add the white chocolate chips, if desired.
Run the ice cream maker for about 20 minutes, following the manufacturer's instruction manual.
Serve immediately or freeze.