If making dulce de leche, place unopened can of milk in a medium pot filled with water (can should be completely submerged); cover and let simmer 2 hours. Remove can and let cool. Open can and transfer contents to a large bowl. Stir in 1/2 tsp. salt and let cool at least 10 minutes.
If using store-bought dulce de leche, combine with 1/2 tsp. salt in a large bowl.
Using an electric mixer on high speed, beat cream in a large bowl until stiff peaks form, about 3 minutes. Stir about 1/3 of the whipped cream into dulce de leche until just combined. Gently fold remaining whipped cream into dulce de leche mixture, then pour into 9x5" loaf pan. Cover with plastic wrap and freeze until firm, about 6 hours.