Heat water with salt over high heat and bring to a boil.
Stir in polenta and reduce to a simmer. It’s important at this point to keep the mixture moving using a strong wooden spoon.
Stir for about 30 minutes — the mixture will thicken immediately, but the consistency and thickness will continue to develop during the 30 minutes. It’s important to keep stirring, don’t give up. (I wanted to give up. I wanted so much to give up. I ended up having my husband help out. He's made polenta before and never stirred for thirty minutes. He didn't see the point until it was all done and the consistency was perfect! If you prefer more of a soft polenta, then I give you the OK to quit.)
After 30 minutes, stir in butter until blended.
Pour the polenta into a greased baking dish and allow to cool for at least 10 minutes. The polenta will become firm and can be sliced at this point.
The polenta can be refrigerated or frozen for later use.