In a medium saucepan, combine ¼ cup sugar, 3 Tablespoons cornstarch, and a pinch salt (if using). Stir in 1 cup heavy cream.
Gently massage 1 whole vanilla bean, then cut lengthwise to expose the vanilla pulp. Add the pulp and the bean to the saucepan.
Heat mixture until warm, but not boiling.
In a small bowl, whisk 3 large egg yolksemper them into the cream mixture by slowly whisking a ¼ cup warm cream into the egg yolks.
Whisk the warmed egg yolk mixture into the saucepan. Return saucepan to medium heat and simmer until thickened.
Remove from heat and stir in 2 Tablespoons butter Cover with plastic wrap to avoid the custard from forming a skin on the top.
Refrigerate for 2 hours before using to fill cake layers or cupcakes.