To small saucepot over medium-high heat, add dulce de leche and coconut milk. Bring mixture to boil, stirring to incorporate coconut milk; keep warm.
Heat greased grill, grill pan or medium skillet over medium-high heat. Sprinkle plantains with cinnamon. Cook, flipping once, until golden brown and heated through, 3-5 minutes. Slice plantains into ¼”- thick rounds.
Mix plantains into dulce de leche mixture until coated in sauce. Serve over ice cream. Sprinkle with walnuts, if desired.
Treat yourself, stat, to this Sweet Plantain Sundae with Coconut Caramel Sauce by making the topping ahead of time. To do, cook and slice the plantains, stir them into the coconut-caramel sauce, then cool. Store, covered, in the refrigerator for up to 1 week. To reheat, transfer plantain mixture to microwave-safe dish loosely covered with plastic wrap. Microwave, on high power, until heated through, about 1 minute. Spoon over ice cream.