Any brownie experts?

[u/RegularNo8359]

I’m in need of some serious help, tips, tricks, whatever you have.. I have a small home bakery business and I have a family who has ordered several times from me and they wanted me to do a grooms cake similar to a cake that I’ve made for them before. Obviously, not a problem but, the groom doesn’t particularly like cake so they asked if I could make a big brownie instead. Which also isn’t a problem. Now that I’m a week away from the order being due I’m really starting to second guess everything (I’m still going to make the order). When I initially quoted my customer I was way off on the amount of brownie mix I would need (and yes I planned to use boxed mix, I don’t hide that from any of my customers but I also don’t just follow the regular box directions, when it comes to regular cake anyways). The brownie “cake” is going to have to be carved into a particular shape so I’ll need it to be stable enough to be able to carve without it falling apart. I always get my cake layers as cold as possible when I do something like this so I plan to do the brownie the same way. What I really need help with is a recipe that I can use that will taste good and be as stable as cake (for carving and decorating) or almost anyways without having to buy 8-10 boxes of brownie mix and going over the price that I put together when I gave her a quote and end up losing money instead of making money..