Place the corn starch (or flour and corn starch) in a pot together with the sugar and the vanilla mixing well and add 4-5 tbsp of cold milk and 4 egg yolks until you have a mixture that is thoroughly incorporated.

Then add the piping hot milk.

Stir vigorously so that it doesn't form any lumps.

Place over low heat stirring constantly in order for it to thicken and get firm. Cover immediately with cling film until it cools to prevent from forming a skin.

When it cools add the whipped cream to the custard or 4 tbsp of butter when you remove from heat.