Place a 2 quart freezer safe container into the freezer to chill while you prepare the ingredients
Combine 20 ounces of sweetened condensed milk, 1 1/2 tablespoons of vanilla extract and 1/2 teaspoon of salt in a medium bowl
Whip 2 3/4 cups of cold heavy cream in a large bowl until stiff peaks have formed
Add 1/3 of the whipped heavy cream into the sweetened milk mixture and gently fold to combine
Repeat the process (1/3 of the whipped heavy cream at a time) until all of the ingredients have been combined
Transfer the batter into the chilled 2 quart container, cover tightly with plastic wrap and place into the freezer for 2 hours
Melt 1 cup of dark chocolate chunks
Drizzle 1/3 of the melted dark chocolate over the partially frozen gelato and let it sit undisturbed until the melted chocolate has frozen
Break the chocolate up and stir it down into the gelato
Repeat the process until all of the chocolate has been incorporated into the gelato
Cover the container tightly with plastic wrap and return to the freezer
Freeze the gelato for an additional 4 hours (stirring a few times over the 4 hours to bring the frozen parts into the center to help the gelato freeze consistently)
Serve chilled
Store in the freezer, tightly covered with plastic wrap