Preheat the oven to 150°C (300°F). Set up a large deep baking dish that will fit 6 pieces 4-inches ramekins in, then place the ramekins inside and set aside.
To make the caramel sugar base, place the caster sugar into a small saucepan (with thick bottom)over medium-high heat. Keep swirling the pan around as it warms up to help you avoid burning the sugar. Keep moving it (don’t use any spoons, spatulas or any other utensil for this part) until all of the sugar has dissolved and turns a caramel colour. *Warning: you need to move quickly here! Once the caramel colour appears, turn off the heat, as the colour will continue to darken. Divide the caramel mixture equally between the 6 ramekins and leave for around 5 minutes, or until the caramel has cooled and hardened completely. For the creme caramel/custard, add the milk and sweetened condensed milk into a large mixing bowl. Crack in three of the eggs. With the remaining three eggs, separate the yolks from the whites into separate bowls. Add the yolks to the milk mixture as well as the pure vanilla extract, reserving the egg whites for later. Whisk all the ingredients together.
Place a fine sieve over a large bowl, then pour the milk mixture through the strainer. This will remove any lumpy parts of egg yolk from the mixture, and will help ensure the end result is as smooth as possible.
Divide the strained mixture equally between the ramekins on top of the caramel using a measuring jug or ladle. You want to leave about 2cm (just under 1”) of room between the top of the creme caramel mixture and the top of the ramekin. Set aside while you make the sponge.
For the sponge cake, add the reserved egg whites and the caster sugar to the bowl of a stand mixer. Use the whisk attachment to whisk the ingredients together on high speed for 5 minutes, or until firm peaks have formed and the egg whites are marshmallowy in texture.
In a separate large mixing bowl, whisk together the egg yolks (pop the whites in the fridge for an omelette the next day), milk and oil until just combined. Add the flour and whisk again until incorporated. Now for the folding. Slowly add half of the egg white mixture to the egg yolk mixture, and gently fold through with a spatula. Repeat with the remaining egg white mixture, taking care not to over mix.
Gently spoon the sponge mixture on top of the raw creme mixture in the ramekins, dividing it equally between the 6 – it should FLOAT!
Make a bain marie by filling the baking dish with water so it’s one-third of the way up the ramekins. Carefully transfer the baking pan into middle rack of the preheated oven and bake for 40 minutes.
Transfer in the lower rack and bake for another 20 minutes. Put an empty baking tray in the middle layer to block the top of the cake from browning too much.
Remove from the oven and leave on your counter top for 5-10 minutes to cool slightly before placing in the refrigerator for 3–6 hours or until completely cool.
To remove the flan cakes, first pour some boiling water into the tray and leave it for a couple of minutes to warm through the caramel layer slightly. Remove one of the flans from the tray and, using a small sharp knife or palette knife, level off the top of the sponge so it’s flush with the top of the ramekin. Run the knife around the edges of the flan too. Now place a serving plate on top of the ramekin. Holding the ramekin and the plate tightly, flip everything over and lift up the ramekin to reveal the finished flan cake. Repeat process with remaining ramekins and serve immediately.
