Preparation

1. In a large saucepan, combine the whole milk and heavy cream over medium-low heat. With a sharp knife, split the vanilla bean lengthwise; use the blunt edge of the knife to scrape out the "seeds" of the vanilla bean. Stir the seeds and bean pod into the milk/cream mixture. Simmer the milk/cream mixture over low heat for 30 minutes. Remove the vanilla bean pod and discard it or rinse and reserve for another use.
2. Combine the sugar, cocoa, eggs, and egg yolks in a medium bowl; using a hand mixer on medium speed, beat until thickened like mayonnaise. Measure out 1 cup of the hot milk/cream mixture. With the mixer on low speed, add the cup of hot milk/cream to the cocoa mixture in a slow, steady stream and mix until completely incorporated. Stir the chopped chocolate into the saucepan with the hot milk/cream. Stir the egg mixture into the hot milk/cream. Cook over low heat, stirring constantly, until the mixture thickens and begins to resemble a chocolate pudding.
3. Transfer the chocolate mixture to a bowl and stir in vanilla. Cover with plastic wrap placed directly on the surface of the chocolate mixture, and refrigerate until completely cooled. Pour the chilled custard into the freezer bowl, turn the machine on and let mix until thickened, about 25 to 30 minutes. The ice cream will have a soft, creamy texture. If a firmer consistency is desired, transfer the ice cream to an airtight container and place in freezer for about 2 hours.
4. Remove from freezer about 15 minutes before serving.