I would crumble it up into a blender, add some vanilla ice cream and some milk.
Blend it all together to make a Brownie Milkshake.
cooking.stackexchange.com/users/8499/Jacob G
I've made great rum balls from overbaked brownies before.
Crush it in a food processor (or just smash it by hand), toss it all into a big bowl, add just enough rum to make it pliable, roll in some cocoa powder and/or powdered sugar, and refrigerate.
cooking.stackexchange.com/users/41/Aaronut
You could also try breaking it into small chunks and soaking it with espresso or a liqueur like cointreau or amaretto.
That should soften it up nicely and would taste great made into an ice cream sundae or trifle.
cooking.stackexchange.com/users/7012/1796foods
I'm afraid you can't do a lot about it now.
The only thing that might work, is to put it in the microwave for a couple of seconds, right before you eat some.
Perhaps putting some syrup on top could help too.
But I guess this would be more helpful with the mouthfeel than with actually softening your 'brick'.
As for next time, if you did like the taste and want to use the same recipe, use a smaller baking tin.
Also check for doneness 5 min before it should be done (by inserting a toothpick in the middle).
That way you can have an estimate how much longer it should be in the oven.
Or as Jay suggested, look for a recipe that tells you the size of the baking tin.
cooking.stackexchange.com/users/4580/Mien
Here's a post where somebody crushed up their brownies and made truffles from them! http://runwithforest.tumblr.com/post/12744781116/how-to-save-burnt-brownies-by-turning-them-into
cooking.stackexchange.com/users/22721/Cocoa Monster
I have a fudge brownie recipe that I followed pretty much to the letter. I think when I baked it, it was too thin; it came out hard, like a brick, and it's pretty much inedible. I'm also afraid someone left it outside / in the fridge exposed, and it dried up more.  Is there anything I can do to soften it and make it edible?
Fixing a Brownie Brick
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<brownies>
cooking.stackexchange.com/questions/21531
cooking.stackexchange.com/users/5714/ashes999
You can blend or crush the brownies to make a milkshake, rum balls or truffles, soak chunks in espresso or liqueur or just try heating it for a few seconds in the microwave before eating. You can blend it up with some milk to make a milkshake, make truffles or rum balls.
Blend it up with some milk and make a brownie milkshake. Try making rum balls. Soak in espresso or amaretto. Microwave for a few seconds before eating. Try making truffles.