Notes: Garnish servings with fresh mint sprigs if desired.
In a 3- to 4-quart pan, combine milk, sugar, chopped mint, lemon peel, and vanilla bean (if using extract, add in step 3). Stir over medium heat just until sugar is dissolved, 1 to 2 minutes.
In a bowl, beat egg yolks to blend. Whisk in warm milk mixture; return to pan. Stir over medium-low heat, scraping pan with a spatula, until custard coats the back of a spoon in a thin layer, 10 to 12 minutes (don't boil).
Pour mixture through a fine strainer into a large bowl; discard lemon peel and mint. Rinse vanilla bean, let dry, and reserve for other uses (if using vanilla extract, stir into strained custard). Nest bowl in ice water and stir mixture often until cold, 20 to 30 minutes. Or cover and chill until cold, at least 2 hours, or up to 2 days.
Meanwhile, in a microwave-safe container, combine chocolate and cream. Heat in a microwave oven at full power (100%), stirring every 30 seconds, until chocolate is soft, about 1 1/2 minutes total. Stir until smooth.
Pour custard mixture into an ice cream maker (1-qt. or larger capacity). Freeze according to manufacturer's directions until mixture is firm enough to scoop, dasher is hard to turn, or machine stops, 25 to 30 minutes.
Scoop softly frozen gelato into bowls, or for firmer gelato, freeze at least 1 1/2 hours or up to 1 week. Spoon chocolate sauce over gelato (if desired, warm sauce in microwave oven at full power for 30 seconds, then stir).