Classic Vanilla Pudding

In a medium saucepan (off heat), whisk together the sugar, cornstarch, and salt. Gradually whisk in the milk to make a smooth mixture with no lumps.
Place the saucepan over medium heat and cook, whisking frequently, until the mixture starts to bubble gently around the edges. Do not let it fully boil. Remove from heat.
In a separate heat-safe bowl, lightly whisk the egg yolks. Slowly ladle about half of the hot milk mixture into the yolks, whisking constantly to prevent scrambling.
Pour the egg mixture back into the saucepan with the remaining milk mixture. Return to medium-low heat. Cook, whisking constantly, until the pudding thickens and coats the back of a spoon, about 2-4 minutes.
Remove pudding from the heat. Whisk in the butter and vanilla extract until fully melted and smooth. For extra-smooth pudding, strain through a fine mesh sieve into a clean bowl if desired.
Pour pudding into individual bowls or a single serving dish. Press plastic wrap directly onto the surface of the pudding to prevent a skin from forming. Let cool to room temperature, then refrigerate until fully chilled and set, at least 2 hours.
Serve chilled, topped with whipped cream, berries, or crushed cookies as desired.