In a large bowl place the egg yolks with half the sugar and whisk until fluffy.
Add 3-4 tbsp of the milk and the corn starch and whisk again until the corn starch has dissolved. Set aside.
Pour the rest of the milk in a large non-sticking pan and add the sugar and the vanilla extract.
Place the pan over medium-high heat and bring to the boil.
As soon as the milk comes to the boil, remove from the heat and gradually ladel in 1/3 of the milk into the egg mixture, whilst whisking.
Add the remaining milk to the pan, then place the pan back on the stove and turn the heat down to medium.
Add the egg mixture into the pan with the remaining warm milk. Whisk continuously, until the mixture has thickened, and is smooth and creamy.
Remove the pan from the stove, add the butter a piece at a time, stirring to melt each time.
Pour the custard through a sieve, into a large bowl, cover with some plastic wrap (the plastic wrarp should be touching the custard, so that a crust will not form on top) and let cool.
Once cooled spread onto the cooled cake (that has been soaked in the sugar syrup).