In this recipe the tartness of the key-limes plays very well off the warm flavor of the cocoa powder and chili powder. Since I first made it I've tweeked it just a little but now I think it's more or less just right. The brown sugar and thyme are reminiscent of Caribbean cooking themes. Mild chili powder may be used if you don't want it to be too spicy; alternatively, if you want a spicier dish use more chili powder! You can store this marinade in the freezer prior to using, but once it has been used it should be discarded to prevent spoiling.