Whisk together the sugar , corn flour and salt in a medium saucepan (thick bottomed). This breaks up the corn flour and prevents it from clumping .

In a glass or bowl , whisk together the egg yolks and the milk until smooth.

Pour the egg yolk- milk mixture into the sugar- corn flour mixture in the saucepan while whisking continuously

Add the butter and heat on medium flame. Whisk continuously.

First the butter will melt and the mixture will come to a boil.

Lower the flame and cook for a couple of minutes more while stirring as the custard thickens.Add any flavouring that you wish now. I added 1 tsp vanilla extract here

Pass through a sieve into a clean bowl. This is optional but removes any bit of cooked egg .

Cover the pastry cream with cling wrap , such that the wrap is touching the surface of the cream . This prevents a film from forming. Chill for atleast two hours .