Begin by pouring the whole milk into a medium saucepan over medium heat. Add the vanilla bean paste or extract, stirring gently to combine. Heat the mixture until it reaches a low simmer, about 5–7 minutes.

In a separate mixing bowl, whisk together the sugar, cornstarch, salt, and egg yolks until pale yellow and slightly thickened, about 2–3 minutes.

Carefully drizzle a small amount of the hot milk mixture into the egg mixture while whisking constantly. Gradually add the remaining hot milk, continuing to whisk until thoroughly blended.

Return the blended custard mixture back to the saucepan over medium-low heat. Stir continuously for about 3–5 minutes until it thickens and coats the back of a spoon.

Once thickened, remove from heat and whisk in the unsalted butter until fully melted and incorporated.

Transfer the custard to a clean bowl, pressing plastic wrap against the surface to cool. Refrigerate for at least 3 hours.