In a heavy saucepan, combine all of the ingredients except the egg yolks and chocolate. Simmer over moderate heat, stirring constantly, 3–4 minutes, or until the sugar is dissolved and the mixture is hot. Do not allow to boil.
Using a hand-held electric mixer or a whisk, beat the egg yolks in a heatproof bowl until light and fluffy. Still beating, very gradually add about 1 cup (250 ml) of the warm milk mixture to the eggs and beat thoroughly; repeat 2–3 times to gently heat the eggs. With the beater running in the saucepan, very slowly stream the egg mixture back into the hot milk. Reduce the heat to medium-low. Stir constantly with a wooden spoon 3–4 minutes, or until the mixture thickens enough to coat the back of the spoon; a candy thermometer should read 180 degrees F (82 degrees C). Do not allow to boil or the eggs may scramble.
Finish ice cream and freeze
Remove the saucepan from the heat source and immediately add the chocolate. Stir briefly to melt then beat with the electric mixer until the chocolate is thoroughly incorporated into the custard mixture, about 1 minute. Allow the mixture to cool, then cover and refrigerate for at least 6 hours or, preferably, overnight. The resulting base will be thick and mousse-like after refrigeration. Process the custard in an ice cream maker according to manufacturer’s directions. Place the finished ice cream in a covered container and freeze to set 2–3 hours before serving.
Prepare ahead note: The custard base for Helado de Chocolate Mexicano should be prepared a day in advance in order to chill thoroughly before freezing. After processing, the ice cream keeps well frozen for 1 week.