Some people simply use another handy container from home, such as a Tupperware container. A length of PVC pipe with a plug in one end and a removable cap on the other also works well and is easy to pack. You're going to be packing poop around in your pack. Make sure the container is easy to wash. Rolling the dough in kitty litter before depositing it into the container aids in clean up later.
Plastic tends to absorb odors more readily than aluminum or other metals.
Make sure the container is strong enough so that it won't explode. When you're packing out your human waste, you'll have to also pack your used TP, or you can burn it. Deposit the brownie onto a flat rock in a spot that is exposed to full sunlight. If there's no privacy there, make your deposit on a flat rock somewhere else and carry the rock to the baking area. Of course, choose your baking area well away from camps, trails, and other areas where humans might possibly want to go.