Fill a large bowl with ice water, then set a fine-mesh sieve over a medium bowl. Set both aside.
In a 3 to 4-quart saucepan, whisk together the sugar, cornstarch and salt. Whisk in the egg yolks until thoroughly combined and all of the dry ingredients have been incorporated. Then, whisk in the milk to combine.
Place the saucepan over medium heat, whisking constantly, until the pastry cream begins to noticeably thicken, about 4 to 8 minutes. When it reaches this point, look for large bubbles to break on the surface. Once they do, whisk constantly for 1 minute, then remove from the heat.
Whisk in the butter and vanilla until melted and completely smooth.
Strain the pastry cream through the fine-mesh sieve into the medium bowl. Immediately press a piece of plastic wrap against the surface of the pastry cream to prevent a skin from forming. Place the bowl of pastry cream into the ice bath to cool for 30 minutes. Transfer to the refrigerator and chill for at least 4 hours or overnight.
Notes
Flavoring Crème Pâtissière
Once you've mastered the classic vanilla, you are ready to take on any flavor your heart desires! Here are some other flavors you might want to try:
Lemon- When you add the vanilla also add the juice and zest of a small lemon to give your pastry cream a tart lemony finish.
Chocolate- Add 4 ounces finely chopped dark or semisweet chocolate to the hot crème pâtissière when you add the vanilla and butter.
Extracts- Try adding ¼ tsp almond or coconut extract to the custard along with the vanilla.
Storing and Making Ahead
Storing- When you first make the pastry cream, it's important to keep a layer of plastic wrap on top of the pastry cream to prevent a skin from forming.
Longer storage- Once fully cooled, keep in an airtight container for up to 5 days.
Making ahead- If you make the crème pâtissière in advance you might notice it gel up in the fridge. Simply smooth it out by whisking it up before using.