In a medium bowl, mix sugar, cornstarch, and salt together, then add to it 1/4 cup milk and whisk smooth.

In a small bowl mix egg yolks, then add them to the mixture and blend until creamy.

In a small saucepan, heat the remaining milk to a simmer. (small bubbles around the edge, not a full boil)

Slowly pour the heated milk into the egg mixture while whisking constantly. Allowing the egg mixture to slowly heat will prevent the eggs from scrambling. (you are tempering the egg mixture)

Place the mixture into the saucepan and heat on medium low heat whisking until it comes to a boil and is smooth and creamy. This takes about 5 minutes.

Remove pan from heat, stir in butter and vanilla.

Place pastry cream into a bowl and cover with plastic wrap, allow it to cool 15 minutes, then put it into the refrigerator to chill.