Hear milk on stove top on low heat just until it comes to a very low boil or steam.

In a bowl, whisk the egg yolks. Then add in sugar, cornstarch, and salt and continue whisking briskly until combined and it is of a creamy yellow and smooth consistency.

Add the tempered egg mixture to the rest of the hot milk on medium heat on stove top and continue brisk whisking until the mixture thickens and becomes creamy and a pale yellow.

Remove from heat and add the butter and vanilla.

Cut warm croissants in half and spread pastry cream. Sandwich with the other half and top with fruit or chocolate shavings (optional).

Transfer custard to a bowl and place over a bowl of ice water to cool for at least 30 to 45 minutes.

Cover the top of custard with Saran Wrap to prevent a skin from forming. Refrigerate custard for several hours before serving. Store in fridge for no more than 2 to 3 days.