I've had brownies with gooey marshmallow swirled in and find them delicious. However, when I try to duplicate this by adding miniature marshmallows to my brownie mix, I find that they just turn into overcooked sugar clumps which don't have a very good flavor or texture.  What's the proper way to add marshmallow to brownies? 
The vast majority of recipes I find call for marshmallows to be added at the last second, and cooked only long enough for them to melt. Usually then a heavy frosting is added to top the whole thing, giving the illusion of marshmallow inside the brownie. Find an example here.  I've also seen really "wet" brownies with lots of included chocolate chips and butterscotch, which contain non-disgusting looking marshmallows. I've never personally tried these because they seem overly sweet, but they're the only ones I've seen that contain marshmallows that seem to retain elasticity after a normal baking period. See examples here and here.  For my money, I'd probably just sprinkle some marshmallows over the top of regular brownies during the last 3-4 minutes of baking. Quick, simple, not overcooked. 