In a large bowl, whisk together ½ cup of milk, the egg yolks and cornflour until smooth and fully combined.
Pour the remaining 1 ½ cups of milk into a heavy based saucepan, and add sugar and cocoa, then heat on low-medium heat until steaming, stirring regularly to dissolve the sugar. Remove from heat.
While whisking, very slowly pour the hot chocolate milk mixture into the egg mixture in a very slow but steady stream. Don’t pour it too quickly or the heat will scramble the eggs.
You now have a chocolate custard base. Return it to the saucepan. Heat over medium heat, stirring with a whisk constantly, until you notice it getting quite thick and big bubbles are starting to break the surface. This will happen quite quickly after about 5 minutes so it’s important to keep gently whisking increasing intensity as it gets thicker. It can turn out lumpy if you don't.
Let it boil for a minute, stirring gently with the whisk, then remove from the heat.
Add the finely chopped chocolate, butter, vanilla and salt. Stir until it is completely melted and combined.
Swap to a silicone spatula and pass the custard through a strainer into a clean bowl, then press plastic wrap to the surface and allow to cool to room temperature.
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Notes
Tablespoons: I use a standard Australian 20ml tablespoon (equal to 4 teaspoons). Check yours before measuring.
Store the pastry cream in a container or bowl with plastic wrap pressed to the surface, in the fridge for 3-4 days.
Chill thoroughly then give it a whisk before use.
Yields roughly 2 cups. This will fill 10-20 choux buns depending on size and whether you cut them open to fill or not.