Place a 9"x 5"x 3" metal loaf pan or other similarly sized container in the freezer.
Whisk together the condensed milk, vanilla, and salt in a large bowl.
Place the cream in a separate bowl. Use an electric mixer with a whisk attachment to whip the cream at medium-high speed until soft peaks form. Add the sour cream, and mix just until combined.
Fold about 1 cup of the whipped cream mixture into the condensed milk mixture. Transfer that mixture to the whipped cream, and gently fold until combined. Stir in the cookie crumbs. If you like, reserve a tablespoon or two of crumbs for the top of the ice cream.
Transfer about half of the mixture to the chilled loaf pan. Dollop about half of the dulce de leche over the top of the ice cream, and use a thin knife or spatula to swirl. Add the remaining ice cream mixture and repeat the addition and swirling of the remaining dulce de leche. If you reserved any cookie crumbs, sprinkle them over the top of the ice cream.
Cover tightly and freeze for 5 hours, or until the ice cream has a scoopable texture.
Notes
Look for dulce de leche alongside other spreads, in the international section, or with the ice cream toppings.