In a saucepan, bring the milk and hazelnut paste to a boil over medium heat until the mixture only just comes to a boil and let it steep.
While the milk is heating, whisk the egg yolks, corn starch and sugar together in a bowl until pale, slightly fluffy and smooth.
Pour half of the hot milk in a slow, thin stream into the egg mixture, whisking constantly.
Make sure the milk isn't boiling hot and you're pouring only half so the eggs can't temper and not cook and curdle.
Return the mixture to the saucepan. Cook over medium heat, with constant whisking, until thickened and it comes to a boil. This will take about 2-3 minutes.
Once the mixture comes to a boil, cook for about 1 minute more.
Remove the pastry cream from heat and add the butter. Stir until the butter has melted and fully incorporated into the pastry cream.
Transfer the finished pastry cream into a flat tray lined up with plastic wrap.
Cover with a sheet of cling film (plastic wrap), and make sure that the cling film is in direct contact with the surface of the pastry cream – this will help to prevent skin formation.
Allow to cool completely to room temperature.
Refrigerate the pastry cream until needed.
Once you’re ready to use the pastry cream, give it a thorough whisk to smooth it out.
It will firm up and set slightly during cooling and chilling, and it might look a bit rubbery initially, but it will smooth out again after a good whisk.
Place it in airtight container and place in fridge until ready to be used.
Bon appétit!