Transfer polenta to a double boiler, set over simmering water for at least 1 hour or up to 3 hours. Wrap the top of the polenta by covering it with plastic wrap. This will prevent the polenta from developing a skin. If you don’t have a double boiler, you can make do by setting the polenta pan on a small, oven proof ramekin centered inside a wider, deeper pot, and surrounding it with just simmering water. Cover pan as above.