Dulce de leche is not caramel sauce and not toffee. It is milk and sugar cooked down slowly until it turns the colour of a penny and thickens enough to hold its shape on a spoon, and it is the single most Argentine thing you can put inside a pastry.
The medialunas are filled after they are baked and glazed, not before. Filling a laminated pastry before it goes in the oven steams it from the inside; filling it afterwards keeps the outside crisp and the syrup glaze intact.
These are sweeter than the plain ones, so one or two is usually plenty per person. If you are ordering for people who have never had a medialuna, the caja mixta puts these next to the plain ones in the same box. The pastry itself is explained in what is a medialuna, and how ordering works covers collection.
