1. Prepare a mold (15x15cm square), insert parchment paper. (Paper must be higher than the mold, by 3 to 5cm). Cover the bottom of the mold with 2 aluminium layers. Set aside.
2. In a cooking bowl, add the oil and unsalted butter. Microwave 2 min at 600w to reach about 80℃. Add the flour, using a cooking sieve and mix well with an egg whisk. Set aside.
3. In a small cup, pour the milk and add the vanilla extract. Mix and microwave 20 sec at 600w to reach about 40℃. Then add it to the cake mixture in one go. Mix well.
4. Add egg yolks in 3 to 4 times. Mix lightly at each addition. Set aside.
5. In an another bowl, add white egg and whisk with an electric hand mixer. Once it starts getting harder, start adding the sugar in 3 to 4 times. Whisk until medium peaks form.
6. Add 1/3 of the egg white to the egg yolk mixture, mix well. Add another third, mix gently together. Finally add all the remaining white egg. Mix with a rubber spatula until egg whites and yolks are well combined.
7. Transfer into the prepared mold and flatten the surface with a rubber spatula. Remove bubbles using a chopstick.
8. In a oven-safe pan, over a folded cloth, place the cake mold. Add warm water (about 40℃), 2 to 3cm deep, and bake in a preheated oven (without fan) at 150C° for 1h ~ 1h 10mn.
9. Drop from about 10cm height to prevent deflating. Unmold and transfer onto a wire rack. Gently remove the parchment paper.
10 Cut in half the cake and slice to serve! You can also sandwich whipped cream in it and fruits to enjoy :)