You can freeze unbaked brownie mix three ways. If you simply want to extend the shelf life of your unmixed, unbaked brownie mix, store the mix itself in the freezer. This will prevent the flour from becoming rancid if you plan to store the brownie mix for more than 12 to 18 months. If you want to prepare the brownie mix ahead of time and freeze the unbaked batter, you can store it either in a plastic freezer bag or in a disposable brownie pan. If you decide to store the batter in a freezer bag, set it on a cookie sheet in the freezer until it is solid. This will cause the batter to freeze in a flat rectangular shape that is easy to stack and store. If you store the batter in a disposable pan, wrap it in aluminum foil to prevent freezer burn.