MAKING THE CARAMEL - Sprinkle the sugar over the surface of a stainless steel pan or non-stick pan. Make sure the sugar is sprinkled in a thin layer so that it's easier for it to caramelize, and also make sure there's no water in the pan and on utensils.
Heat over medium heat, taking care not to introduce water into the sugar. Stir the sugar as it melts with a heat-proof spatula. The sugar will stick to the spatula, but because it’s non-stick, it'll come off easily too.
Continue heating the sugar, while stirring frequently, until the sugar turns into caramel (a golden to amber color) (Please be careful NOT to touch the sugar or the pan as they will be very hot at this point!).
When the color is amber, add the ½ cup of warm cream into the pan. PLEASE STAND BACK AS YOU ADD THE CREAM AT THIS STAGE, because the pan is extremely hot and you don't want splash back. Stir soon after adding the cream, to dissolve the sugar. This will take a few minutes.
When the caramel has mostly dissolved in the cream, add the rest of the milk, salt, and vanilla into the pan. Heat the milk until it’s nearly boiling, while stirring, to dissolve the rest of the sugar. Now you have a caramel flavored milk base for the custard.
While the caramel milk is heating (to dissolve all the caramel), in a separate bowl or small jug, add the eggs and cornstarch (cornflour), and whisk until you have a smooth mixture.
Temper the egg mixture by adding a little of the hot caramel milk in a slow and steady stream, while whisking the eggs constantly. After adding enough caramel milk to warm up the mixture, pour the egg-milk mixture back into the pan with the rest of the caramel milk.
Heat the egg-caramel milk mixture while whisking/stirring constantly, until the milk starts to thicken. Stir well to make sure the eggs don’t curdle and stick to the bottom of the pan as the custard heats and thickens. I like to switch between a whisk and a spatula to make sure the custard stays smooth while it’s thickening.
Stir the custard until it comes to a boil (the first big bubble to breakthrough the surface from the custard). When the custard starts to boil, remove it from the heat. If there are clumps, pass the custard through a fine sieve to remove them.
Add the butter to the hot custard and mix until the butter has incorporated into it. Pour the custard into a bowl, and immediately cover the surface with plastic wrap, making sure the plastic wrap is touching the whole surface of the custard.
Allow the custard to cool down to room temperature, then store it in the fridge for a few hours until chilled, and it's ready to be mixed with the cream.