I can only find "sugar that is brown," not "brown sugar" that I use in the states. Anyone else have the same problem, or a creative solution?
No one here seems to know what molasses is either ... bad start.
The sugar that is brown is likely beet sugar that's had some molasses (so someone somewhere knows what it is) added to it because beets contain no molasses. In a pinch, it will do.
I am unfamiliar with Buenos Aires but if you can find Black Treacle it is basically molasses. It's a British product but I have seen it in a number of grocery stores serving communities that are not British.
Perhaps FrancesRenHuang, a food52 member in Buenos Aires, would know. You should be able to contact her through her food52 profile. She's a great resource!! (And such a helpful person, too.) ;o)
Across the way in Chile, we've made great use of Chancaca. It's sold in bricks that we break up and use 1:1 for Dark brown sugar.