Add the milk, cream and vanilla extract to a large pan and heat on high until it just starts too bubble, then turn off the heat.
In a large bowl, mix the cornflour, egg yolks and caster sugar. Pour in a little of the boiled milk to the egg mixture and whisk quickly before the egg tries to scramble. Add another couple of splashes in and whisk. Now add the rest of the milk, and whisk. Give your milk pan a quick wash and dry (or grab a fresh pan), then pour the milk and egg mixture back into the pan. Heat on a medium heat stirring with a whisk all the time until the custard is very thick. It will take about 5-6 minutes and will thicken quite suddenly towards the end. Once thickened, turn off the heat and stir in the butter until incorporated. Allow to cool for 10 minutes, then place some clingfilm/plastic wrap directly on top of the custard - so it's touching the top of the custard - and leave to cool completely (this prevents a skin forming on the custard).
Next make the biscuit base:
Line a 20cmx20cm baking tin with baking parchment.
Put the biscuits into a large bowl and crush into crumbs using the end of a rolling pin, then mix in the melted butter until thoroughly combined. Spoon the mixture into the baking tin and push down so you have a tightly packed, level layer covering the bottom of the tin.
Spoon in the cooled custard mixture and flatten with the back of a spoon. Place some clingfilm on top (touching the custard) and place in the fridge for 2-3 hours (or overnight) to set.
Once set, take out of the fridge and remove from the tray (using the baking parchment to carefully lift it out). Place on a chopping board and slice into 10 slices. Wipe the knife after each slice to get a neat, clean slice for each bar.
Sprinkle half a tablespoon of granulated sugar on top of each bar and smooth out with the back of a spoon. Levelling the sugar in this way will ensure even melting of the sugar.
Ensure the bars are on a scorch-proof surface (such as a baking tray) and melt the sugar - using a blow torch on a medium heat setting - until the sugar browns and bubbles rapidly.