Fill a large bowl with ice, and add enough water to fill the bowl halfway. Place a medium bowl in it with a fine mesh strainer nearby along with a piece of buttered wax paper. Set aside.

Place the sugar and egg yolks in a medium bowl and whisk until well incorporated. Add the cornstarch and and whisk vigorously until smooth.

Place the milk in a medium saucepan and heat over medium low heat until just before it begins to boil (it will start smoking). When the milk is hot enough, add 1/3 of it to the egg mixture in a slow steady stream while whisking constantly to temper the eggs. Once 1/3 has been added, add the rest while whisking constantly until combined.

Place the mixture back in the saucepan and cook over medium heat, whisking constantly to prevent lumps, until the mixture begins to boil. Continue to cook for another minute until very thick, whisking constantly. Remove from heat and whisk in the vanilla and softened butter.

Strain the pastry cream through the sieve into the bowl over the ice bath using a rubber scraper to gently press it through as needed.

Cover the pastry cream with the buttered wax paper to prevent a skin from forming and allow to cool at room temperature. Once cooled, place in the refrigerator for a minimum of 1 hour and up to 2 days in advance.