In a medium saucepan combined milk, 1 cup cream and fresh mint leaves. Heat over medium heat and bring just to a gentle bubble, about 5 minutes. Remove from heat and allow mixture to steep, 15 minutes.
Meanwhile in a bowl, whisk together remaining 1/2 cup heavy cream with granulated sugar, corn syrup and egg yolks until slightly pale and fluffy, about 1 - 2 minutes.
While whisking egg yolk mixture, slowly add about 1/2 cup warm milk mixture to egg yolk mixture (don't strain leaves yet). Add egg yolk mixture to milk mixture in saucepan and return to medium heat.
Cook mixture, stirring constantly until thickened, about 5 - 8 minutes (mixture should coat the back of a wooden spoon. A light bubble is fine but don't let it boil). Remove from heat and pour mixture through a fine mesh sieve into a heat proof bowl.
Using a rubber spatula, rub mint leaves back and forth in over sieve to strain oils and natural flavors from leaves into bowl (also scrap bottom of sieve once done and add that to bowl), discard leaves.
Mix in a few drops green and blue food coloring if desired (about 4 green/2 blue) and mint extract. Cover with plastic wrap pressing directly against surface of mixture to prevent a skin from forming. Transfer to refrigerator and allow to thoroughly chill through, about 4 - 5 hours.
Once fully chilled, process mixture in an ice cream maker according to manufactures directions, adding in chopped chocolate during last few minutes of processing. Transfer mixture to an airtight container and freeze until nearly firm.**
Recipe Source: adapted slightly from Williams Sonoma