We always use a Bakepacker.  It is essentially an aluminum circle with an aluminum grid inside the cirlcle about 1" tall.  It fits inside a pot, fill water to top of the grid and bring it to a boil.  We mix up the brownies in a plastic bag and lay it on the grid.  Moist, delicious browines in about 20 minutes.  They also make great muffins for breakfast.  Its an esstial on all our trips.