Heat the milk and vanilla until hot but not boiling then turn off the heat.
5 cups whole milk, 1 teaspoon vanilla extract
Meanwhile, add the egg yolks to a large bowl (big enough to hold all the milk) and add the sugar, mix until combined. Add the cornstarch and mix together until smooth.
10 egg yolks, ⅔ cup sugar, ½ cup cornstarch
Whilst constantly whisking, gradually add the milk to the egg yolk mixture until all combined. Transfer everything back to the pot and turn on a low heat.
Stir the milk and egg mixture until thickened, it should be thick enough to coat the back of the wooden spoon without dripping.
Transfer just over half of the pastry cream (about 3 cups/690g) to a bowl keeping the remaining in the pot. With the heat off add the chopped chocolate to the pot and stir until melted and completely combined with the remaining pastry cream.
5.5 oz dark chocolate
Transfer the chocolate pastry cream to a separate bowl. Cover both bowls with plastic wrap (cling film) ensuring that it’s directly on top of the pastry cream to stop a skin from forming, allow to cool completely.