Take two tablespoons of the sugar and put into a bowl along with the egg yolks, flour and cornflour - whisk to combine.
½ cup White Sugar, 5 Egg Yolks, 1.5 tablespoon All-purpose Flour, 3 tablespoon Corn starch
Put the remaining sugar into a suacepan with the milk and vanilla, slowly warm over a medium-low heat
2 ½ cups Whole Milk, 2 teaspoon Vanilla Extract
When it begins to get warm, drizzle a small amount of warm milk into the egg yolk mixture and whisk, then add a little more, repeat until everything is together in the bowl, then pour the whole mixture back into the saucepan. It's important to do this slowly and gradually so you don't scramble the eggs
Keep heating the mixture, whisking constantly until it begins to boil. Boil for one minute or until you see thick bubbles bursting on the surface. Remove from the heat
Pass the pastry cream through a sieve into a bowl to make sure there are no lumps
Whisk in the butter one small piece at a time, making sure it melts into the mixture completely before adding the next piece
Cover with cling wrap (make sure the cling wrap is touching the surface of the pastry cream to prevent a skin forming) and refrigerate for at least two hours. I like to make this the day before and let it sit overnight, but it can be used after 2 hours.
Notes
Make sure you whisk constantly while your custard is on the heat - if you let it sit you will end up with lumps of egg or burnt pastry cream!When your pastry cream is ready, you will be able to coat the back of a spoon and run your finger through it to make a line - the line will not fill in, it should stay visible. This means your pastry cream is cooked enough. If left to develop overnight, the custard has a much richer flavour - if you have time I highly recommend this!