Whats the best way to preserve the softness of brownies?

It depends on the recipe you are using.

There are different "kinds" or textures of brownies.

I keep them in a large tupperware tub, but they dont usually stay around long enough to go hard !

Don't cut them until you use them and keep them in an air tight container.

Cook them til they are not quite done, then keep them tightly wrapped or covered.

It depends on the recipe you are using. 

There are different "kinds" or textures of brownies. 

Some are "cakey" these will get "stale" or drier faster than other kinds. 

Some are "fudgy" these will stay moist longer. 

Some are sort of a blend of the two. 

The important thing no matter what kind of brownie you are making is DO NOT OVERBAKE. 

Take them out of the oven before they "test done" since brownies are supposed to cool in the pan there is about 30 minutes of "carry over baking" where the brownies will continue to bake even after you remove them from the oven. 

The larger you keep them, the more intact, the less surface area there is to dry out. So if you want to keep them for a few days wrap them whole just as they came out of the pan.  Or cut them into as few pieces as you an before you wrap them 

AIRTIGHT container. The less air that gets to them the more moist and soft they will stay. 

Do not refrigerate or freeze both dry out foods. If they are to be kept for an extended time freezing is better than refrigerating. 

Wrap tightly. keep in the fridge. 

Don't over bake and use a good recipe.

🥴😊 Keep them at room temp and sealed.

Also tweak the ingredients when baking to make them cakey.

Its usually another egg. More water or oil. Or flour. I cant remember.