Wash and lightly dry your mint leaves.
Put the egg yolks in a heat-proof bowl, and whisk until well-combined.
In a medium-sized pot, combine whole milk, sugar, and mint leaves over medium heat until the sugar has dissolved. You really want to "bruise" the mint up. (If not using mint leaves, just combine milk and sugar. Extract will be added later.)
Take 1/3 cup of the sugar/milk mixture (try to keep it free of the mint leaves), and slowly, very slowly, dribble the hot mixture into the egg, whisking very quickly the whole time.
Return the egg/milk mixture to the pot, and whisk to incorporate. Cook for an additional 1-2 minutes, making sure to not let the mixture boil.
Set up a fine mesh sieve over a heat-proof bowl. Pour the contents of the pot through the sieve. Add the heavy cream to the mixture in the bowl, and stir well. Put the bowl in the fridge for 6 hours, or until completely cold.
If you are using mint extract, add it here before putting it into the ice cream machine. Taste and add more extract if needed.
With your ice cream maker running, add the base to the machine, and churn until it starts to resemble ice cream. Add the chocolate to the mixture, and continue to churn until thick and creamy.
Remove the ice cream from the machine and put it in a freezer-proof container with a lid. Freeze for at least 2-4 hours before serving.