Line a metal loaf pan with parchment paper and set aside. (Lining with paper is optional, but it helps protect the pan from getting scratched by the ice cream scoop later. Avoid using a glass or plastic pan for this - see notes).
Pour two cups cold heavy cream into a large mixing bowl. Beat with a mixer on high until soft peaks begin to form.
Add vanilla extract and pinch of salt (if using).
Using a spatula, fold in sweetened condensed milk, mixing gently until everything comes together.
Pour half of the cream mixture into the prepared loaf pan. Drizzling with a spoon or spatula, top with ½ of the dulce de leche. Gently pour remaining ice cream base on into the loaf pan. Repeat drizzling with remaining dulce de leche.
Using a knife, spoon or spatula, gently swirl the ice cream base and dulce de leche together but do not over mix.
Place loaf pan in the freezer for 8-12 hours, or overnight, until firm.
Allow ice cream to set out at room temperature for about 5 minutes, prior to serving.