Coat 9-inch tart pan with removable bottom with cooking spray. In medium bowl, mix ground cookies, butter and applesauce; press onto bottom and up sides of pan. Bake at 350 degrees until golden, about 30 minutes. Cool crust in pan on rack. In heatproof bowl, whisk eggs, cornstarch, sugar and salt. In blender, puree watermelon; strain into a medium saucepan, pressing on solids. Bring watermelon juice to simmer over medium-high; gradually whisk into egg mixture. Return to saucepan. Simmer over medium-low, whisking constantly, until mixture thickens, about 1 minute. Spread evenly in crust. Tent tart with foil and refrigerate until cold, about 4 hours. Top with whipped cream.