1 tablespoon plus 1/2 teaspoon unflavored powdered gelatin
Plain low-fat yogurt and candied rose petals, for serving (see Note)
In a small saucepan, combine the cranberries with the sugar and water and bring to a boil. Simmer over moderate heat, stirring occasionally, until the cranberries soften, about 5 minutes. Mash the cranberries with the back of a spoon to make a thick sauce. Transfer the sauce to a large bowl and refrigerate, stirring often, until cool, about 15 minutes. Stir 3 cups of the buttermilk into the cooled cranberry sauce.
In a small saucepan, sprinkle the gelatin over the remaining 1/2 cup of buttermilk. Let stand until the gelatin has dissolved, about 5 minutes. Heat the mixture over low heat, stirring a few times with a rubber spatula, until the buttermilk is just warm and the gelatin is melted, about 1 1/2 minutes; fold into the cranberry mixture.
Pour the panna cotta into 8 glasses. Cover and refrigerate for about 2 hours, until they are set. Top each with a small dollop of yogurt and a few candied rose petals and serve.
The panna cottas can be refrigerated for up to 3 days.