Boil the water, add rice and vanilla bean or vanilla extract. Cook until the rice is soft.
Cool, remove the vanilla bean (if using) and transfer the mixture to a blender or food processor. Blend until very smooth.
Allow this mixture to stand for at least 30 minutes and then carefully pour the top layer of the liquid into another container. Be careful not to let the sediments in. Else, you may strain it through a very fine sieve and decant the liquid at the top when serving. Refrigerate. Serve cold. It can be kept for at least 48 hours.