In a medium saucepan, whisk granulated sugar, cornstarch, and a tiny pinch of salt (before putting it on the stove). Add egg yolks, milk, and cream. Whisk to combine.
Heat the egg mixture over medium-low heat while whisking, until it begins to boil. This will take about 15 minutes. Once boiling, let bubble for 1 minute while continuing to whisk.
Remove from the heat and stir in butter and vanilla.
Pour into a medium bowl and cover with plastic wrap allowing the wrap to rest on the surface of the warm custard.
Refrigerate for at least 2 hours or until chilled.
