Special equipment: Several ceramic or silicone ghost-shaped molds
Make the Panna Cotta: In a very small saucepan sprinkle gelatin over water and let stand about 1 minute to soften. Heat gelatin mixture over low heat until gelatin is dissolved and remove pan from heat.
In a large saucepan bring cream, half-and-half, and sugar just to a boil over moderately high heat, stirring. Remove pan from heat and stir in gelatin mixture and vanilla. Divide cream mixture among the ghost molds and cool to room temperature. Chill molds, covered, at least 4 hours or overnight.
Make the Sauce: In a small saucepan simmer juice, fresh or frozen cranberries, and sugar, stirring occasionally, until cranberries are softened, about 7 minutes. In a blender, puree mixture until smooth (use caution when blending hot liquids). Pour sauce through a sieve into a small bowl, pressing hard on solids, and discard solids. Cool sauce. Sauce may be made 2 days ahead and chilled, covered.
Run a thin knife around edge of each mold. Dip molds, 1 at a time, into a bowl of hot water 2 seconds and invert each panna cotta onto a plate. Serve panna cottas with the sauce spooned around the sides.