Prepared Dulce de leche is available in Latin food markets but homemade tastes much better and is easy to do. For homemade Dulce de Leche, prepare filling several hours or the day before. Spoon condensed milk into the top of a double boiler and set over simmering water. Stir occasionally, allowing milk to heat for several hours on very low heat. Eventually, the milk will thicken and turn a deep butterscotch color. Cool well. It will thicken further. Refrigerate until needed.