Sprinkle gelatin over cold water in small cup; let stand 1 minute to soften. Add boiling water; stir until gelatin is completely dissolved. Cool slightly.

Stir together sugar and cocoa in medium bowl; add whipping cream and vanilla. Beat on medium speed of mixer, scraping bottom of bowl occasionally, until mixture is stiff. Pour in gelatin mixture; beat until well blended. Store in the refrigerator while you make the pastry.