Heat the oven to 180C/160C fan/gas 4. Tip the caster sugar into a large saucepan along with 2 tbsp water, and cook over a high heat without stirring until the sugar starts to dissolve. Once the sugar has started to dissolve, you can swirl the pan occasionally until you have an evenly coloured, golden caramel, about 5 mins. (It's crucial that the sugar dissolves completely to achieve this.) As soon as the mixture has turned into caramel, tip it into a 20cm flan tin (it should not be a loose-bottomed tin), tilting the tin back and forth to completely coat the base. Set aside to cool.

Whisk the eggs and vanilla together in a large bowl, then vigorously whisk in the condensed, evaporated and whole milks until well combined. Pour the custard into the tin and cover with foil, sealing well at the edge. Put the tin in a high-sided baking tray (about 5cm deep) and pour boiling water from the kettle into the tray so it reaches about halfway up the side of the tin. Transfer the tray in the oven and bake for 50 mins.

Carefully remove the tin from the tray and remove the foil. Put the tin on a work surface and leave to cool for about 30 mins. Once just warm, cover and chill for at least 6 hrs, or overnight.

When you're ready to serve, run a cutlery knife around the edge of the tin, then put a large serving plate (use a plate with a lip, as the caramel will run out) over the tin and invert quickly. Cut into wedges to serve.