Heat the heavy whipping cream in a medium saucepan over medium heat until it begins to simmer, stirring often.
In a medium mixing bowl, whisk together the egg yolks, sugar, and salt. Add the cornstarch and whisk until well combined.
Once the cream is simmering, very slowly whisk it into the egg and sugar mixture a small amount at a time. Be sure to whisk as you add the cream to incorporate it quickly.
Return the pastry cream to the saucepan, add the butter and vanilla extract, and cook over medium heat until thickened, stirring constantly. In 1-2 minutes the cream should reach a pudding-like consistency.
Pour the pastry cream into a small bowl, cover with plastic wrap, and place in the refrigerator to chill.