Heat heavy cream in medium sauce pan over medium heat until simmering, stirring occasionally. In a separate bowl, whisk egg yolks, sugar and salt together. Add cornstarch into mixture, continue whisking until it is a pale yellow and thick.
When cream reaches full simmer, slowly whisk it into yolk mixture. Return mixture back to sauce pan, continue cooking over medium heat, whisking constantly until thick and glossy, about 1-2 minutes. Take off heat whisk in butter and vanilla. Transfer pastry cream to a bowl large enough to hold it, take some plastic wrap press directly onto surface, refrigerate until cold and set, at least 2 hours.