from The Perfect Scoop
2 cups heavy cream
3 tbsp unsweetened Dutch-process cocoa powder
5 oz bittersweet chocolate, chopped
1 cup whole milk
3/4 cup sugar
Pinch of salt
5 lrg egg yolks
1/2 tsp vanilla extract
Warm 1 cup of the cream with the cocoa powder in a medium saucepan, whisking to thoroughly blend the cocoa. Bring to a boil, then reduce the heat and simmer at a very low boil for 30 seconds, whisking constantly. Remove from the heat and add the chopped chocolate, stirring until smooth. Then stir in the remaining cream. Pour the mixture into a large bowl, scraping the saucepan as thoroughly as possible, setting a mesh strainer on top of the bowl.
Warm the milk, sugar, and salt in the same saucepan. In a separate medium bowl, whisk together the egg yolks. Slowly pour the warm milk into the egg yolks, whisking constantly, then scrape the warmed egg yolks back into the saucepan.
Stir the mixture constantly over medium heat with a heatproof spatula, scraping the bottom as you stir, until the mixture thickens and coats the spatula. Pour the custard through the strainer and stir it into the chocolate mixture until smooth, then stir in the vanilla.
Chill the mixture thoroughly in the refrigerator, then freeze it in your ice cream maker according to the manufacturer's instructions.
from same book as above
5 oz semisweet chocolate, chopped
Melt the chocolate in a liquid measuring cup by zapping it in the microwave at 30-second intervals. I think mine took about 1 minute total. At the very last second of churning the ice cream, drizzle in the melted chocolate in thin streams, making sure not to get chocolate on the dasher. You can also spoon some into the ice cream as you layer it in a plastic container, making sure to break up the chunks.