Add the cranberries and water to a small-to-medium sized pot. Cover and place over medium heat and cook until cranberries burst, about 10 minutes (may take a couple minutes longer if you are using frozen cranberries).
Remove from heat, and press the cranberries through a mesh strainer with a spoon. Be sure to scrape the underside of the strainer to collect all the puree. Pour back into the pot and set aside off the heat.
Whisk together the egg yolks and egg in a small bowl. Add to the cranberry mixture along with the sugar and salt, whisking quickly to incorporate (be sure to do this quickly as possible so that the eggs don't cook from the heat of the cooked berries and turn into scrambled eggs!).
Place the pot back over medium heat and whisk continuously for 8-10 minutes, until the mixture has thickened. Stir in the butter until fully combined and then remove from heat.
Pour the cranberry curd into a bowl or jar, cover and chill in the fridge for at least 1 hour before serving.