Babalú makes its ice cream from the plants that grow around here, which in this desert is a short and strange list. Rica-rica is a small aromatic shrub from the altiplano that tastes somewhere between mint and thyme; chañar is the fruit of a thorn tree, sweet and slightly caramel; algarrobo is carob from the same trees that shade the plaza. There is a coca-leaf flavour as well, which is legal in Chile and tastes greener than you expect.
Get one of those rather than the chocolate. The point of the place is that you cannot buy these anywhere else, and after a few days of the same landscape it is a pleasant reminder that things do actually grow here.
There are several Babalú counters within a few blocks of the plaza — they are all the same operation, so take whichever one you walk past. Open late, which in San Pedro means about eleven.