Whisk the sugar, cornstarch, and salt together in a medium saucepan. Slowly whisk in 1 cup of the milk until completely smooth.
Whisk in the remaining milk and heavy cream, then place the saucepan over medium heat.
Cook while whisking continuously until the custard begins to bubble and noticeably thickens. Once bubbling, continue cooking for 1 minute to fully activate the cornstarch and remove any starchy flavor.
Remove the custard from the heat and whisk in the butter and vanilla until silky and smooth.
For warm custard, serve after cooling for about 10 minutes. For chilled custard, press a piece of plastic wrap directly against the surface and refrigerate for at least 3 hours.
