For the ice cream: Whisk together the condensed milk, cocoa powder, vanilla and salt in a large bowl. The mixture will become very thick; set aside.
Whip the cream with a mixer on medium-high speed until firm peaks form, about 2 minutes. Fold about 1 cup of the whipped cream into the cocoa mixture with a rubber spatula until combined, then fold the lightened mixture into the whipped cream until well blended. Pour into a chilled 9-by-5-by-3-inch metal loaf pan, and freeze, covered, until thick and creamy, like soft-serve, about 2 hours. Swirl in any desired mix-ins with a spoon. Continue to freeze, covered, until solid and scoopable, about 3 hours more.
