1. Place the gelatin in a heatproof measuring cup. Stir in the cold water and let stand until it thickens (about one minute). Heat in the microwave for about 30 seconds or place in double boiler to heat and melt the gelatin. The mixture should be clear.
2. In an electric mixer fitted with beater attachment, whip the cream until soft peaks just begin to form. With mixer still running, add the sugar and beat until it stiffens into peaks (overmixing can cause cream to curdle, so watch it carefully). Beat in the vanilla and add the melted gelatin in one shot, and beat until fully incorporated. Refrigerate for half an hour. Spread a thin layer over top of cake and pipe stars all around the perimeter, if desired.
3. Slice with a hot knife for clean, even pieces or use floss to cut nice wedges.