In a large bowl whisk together the sweetened condensed milk, cocoa powder, vanilla extract, and salt.
In a second bowl (preferably a chilled glass bowl) whip heavy cream until stiff peaks form (a hand mixer on medium-high speed will speed this up but it can be done by hand!).
Gently fold heavy cream into the chocolate mixture.
Pour half of the ice cream into a bread loaf pan.
Drop half of the hot fudge sauce by spoonfuls onto the ice cream, gently swirl in with a knife a few times.
Repeat with remaining ice cream and hot fudge sauce.
Cover with plastic wrap and freeze for 8 hours or overnight. Scoop and serve!