Jim Meehan (of PDT fame) created the Dulce de Leche as an homage to rum raisin ice cream. It’s a perfect match for that classic flavor, without being cloyingly sweet.
Meehan uses Pedro Ximénez sherry here, the richest and sweetest type. If you use any other type of sweet sherry, you might want to add a touch of brown sugar syrup to keep the sweetness level up to par.