Freezing brownies as an uncut block is the most effective method for protecting them from freezer burn and oxidation.
To freeze a whole pan, line the pan with parchment paper. Ensure that plenty of paper pokes out above the brownies so it’s easy to grab.
After baking the brownies, let them cool completely. Remove the slab of brownies from the pan. Wrap the whole sheet of brownies tightly with plastic wrap, then wrap them with foil. Then, either place the entire sheet in your freezer as is or put it in a large freezer bag.
While this method keeps the brownies at their tastiest, it may not be convenient if you need individual servings immediately. The sheet of brownies will need to be completely thawed before they can be sliced and served, so this may not be the best option if you're looking for a quick and easy dessert.
How to freeze individual brownies