Place cream, sugar, and butter in a medium saucepan and bring to a boil over medium-high heat, stirring occasionally until sugar is dissolved
Meanwhile, in a small bowl, mix together cornstarch, vanilla, and 1 tablespoon water. Add to cream mixture, bring to a boil, and simmer until thickened, about 1 minute.
Place coconut in the bowl of a food processor. Pulse until coconut is finely chopped. Remove cream mixture from heat and stir in coconut until well combined. Transfer to a large baking dish; let cool.
Cover filling with plastic wrap and chill overnight. Just before using, place mixture in the bowl of an electric mixer fitted with the paddle attachment. Beat until smooth and creamy, 4 to 5 minutes