Yes, it is absolutely okay to cook only half of the box brownie mix and save the rest for later. Brownie mixes typically come in larger quantities than you might need for a single serving, so it's quite common for people to bake only a portion of the mix and store the rest for later use.
To save the remaining half of the brownie mix, follow these steps:
Measure the remaining mix: Use a kitchen scale or measuring cup to ensure you have exactly half of the mix left.
Store in an airtight container: Transfer the unused mix to an airtight container to keep it fresh and prevent it from absorbing moisture.
Label the container: Write the date and the type of mix on the container to help you remember what's inside and to track its freshness.
Store in a cool, dry place: Keep the container in a cool, dry place like your pantry or cupboard. Avoid areas with high humidity as it can affect the mix's quality.
Check the expiration date: If the mix has an expiration date, make sure to use it before that date.
When you're ready to use the mix again, check it for any signs of spoilage or rancidity before baking. If the mix has been stored properly and is within its expiration date, you should be good to go!
Please note that the specific instructions may vary based on the brand and type of brownie mix you are using, so it's always a good idea to refer to the packaging for any additional guidance or recommendations.