Sprinkle gelatin over 3 tablespoons water in a measuring cup, and let stand until gelatin softens. Microwave on HIGH for 10 seconds; stir until gelatin dissolves. Cool 2 to 3 minutes. (Gelatin must be liquid but not warm.) Beat cream in a large bowl at high speed with an electric mixer until soft peaks form. Add powdered sugar, and continue to whip until you can see streaks holding their shape in the bowl.
Add cooled gelatin in a slow, steady stream while beating. Add vanilla, and beat to stiff peaks. Refrigerate until ready to use, or frost cake and refrigerate until ready to serve.