The last couple of times that I have baked brownies, they would come out with a thin layer of oil at the top. I don't know if i'm not giving the butter-sugar mixture enough time to cool down but after I add the egg and chocolate, the batter looks too runny and oily. First Method: Melt butter and sugar together in a pot until homogeneous, add beaten egg then chocolate. Put all of that together with the dry ingredients and bake.  Recently, I tried doing something a little bit different. I've been making caramels, so I kind of experimented with my sugar. New Method: Melt sugar until it becomes a syrup, then add hot melted butter until it becomes homogeneous. Add the eggs, chocolate and put them together with the dry ingredients. This time, the batter looked really good. It was nice and thick.  This time the brownies did not become oily. They were good and fudgy.  My question..does anybody melt their sugar before adding the rest of the ingredients in? I know that for Italian meringue and buttercreams, it is a necessity to melt the sugar, but for brownies? I don't know. It worked for me. 
Doug has the right idea, but ovens trap the moisture, increasing the humidity around the food, decreasing the efficiency of the oven as a drying machine.   For detailed information regarding the physics of drying foodstuffs, I found the following helpful: http://www.nzifst.org.nz/unitoperations/drying1.htm  From these ideas, I would suggest placing the mangoes in a dry ventilated room, with some sort of heat source applied (doesn't have to be much). The combination of the dry air and the heat will speed the evaporation.  