Heat 1 cup whole milk and 2 cups heavy cream1 cup heavy cream with ¾ cup light brown sugar in a medium saucepan. Stir to dissolve the sugar, bring just to a boil and remove from heat.
Place 6 egg yolks in a large bowl and gradually add ¾ cup sugar, beating until beginning to thicken. I do this with a hand whisk, but you can use an electric mixer.
Add the hot milk and cream to the yolk mixture gradually, whisking constantly.
Transfer this mixture back to the saucepan and cook over medium heat, constantly stirring, until it thickens, from 3 to 5 minutes. Don't let it boil. When you draw your finger across the back of the spoon (a wooden or silicon spoon), there should be a clear path that doesn't expand quickly.
Remove from the heat and stir in 3 tablespoons unsalted butter. Stir to dissolve.
Strain this mixture into a medium bowl, add 2 teaspoons vanilla extract, ½ teaspoon salt and the remaining 1 cup of heavy cream.
Cover and refrigerate for 8 hours or overnight.
Pour the cold ice cream mixture into the bowl of the ice cream maker and process according to the manufacturer's directions.
Transfer to a metal pan or freezer-safe container, stir ½ cup semisweet chocolate chips and freeze until solid. Scoop into bowls or waffle cones and sprinkle the remaining ½ cup chocolate chips before serving.