Use a big bowl and whisk yolks, sweetener and corn starch until they become a thick paste, using a handheld whisk. Set aside.
Use a heavy bottom pot, heat milk and cream until it just starts to simmer. Turn heat off.
Pour bit by bit all the milk cream into egg mixture while whisking. Then pour everything back into the pot (I clean it beforehand).
Cook on low medium heat until the mixture thickens, while whisking the whole time. Once thickened, turn the heat off. Add in vanilla extract and mix everything together.
Pour into a bowl and cover with cling film, making sure it touches the pastry cream. Leave to cool to room temperature. Chill for 1-2 hours in the fridge. Whisk before using to get back its silky creamy texture.