Preheat oven to 150 degree C ( 300 degree F). Place the oven rack in the ⅓rd lower rack. Take a 15 x15 cm (6x6 inch) square cake pan. Line with parchment paper so the paper over hangs the cake pan. This way if the cake rises, it doesn’t overflow. Place this cake pan on a large cake pan with high sides, since we will be baking this in a Bain-marie (water bath). Bring a kettle of water to a full boil, set aside.

Now take a mixing bowl which is large enough to mix the cake batter. Place a sieve over it, add in plain flour, salt, cornstarch in the sieve and sift it so the flour gets lightened. Mix the sifted ingredients really well using a whisk. Set this aside.

Take a sauce pan, add in milk, butter and warm it slightly in the stove till butter is melted. You have to heat this on very low heat. It shouldn’t boil. Once butter is melted, remove it from heat and add in vanilla extract. Mix well and set aside.

Now take the dry ingredients which we sifted earlier, pour in the warmed milk and butter mixture into it. Use a whisk to mix it gently so it forms like a thick batter. Now take the 4 egg yolks, add one egg yolk at time into the batter and mix gently. It will get into a smooth consistency. Don’t over whisk. Set this aside.

Now in a clean, dry steel bowl. Add in the egg whites along with cream of tartar. Use an electric beater to whip the egg whites. It will get frothy and airy. Now start adding sugar little at a time and keep whisking. It will get shiny, thick and more airy. Once all sugar is added, beat for few more seconds so the sugar is melted into the meringue. If you try inverting the bowl, the meringue shouldn’t fall off the bowl. This is the right consistency.

Now take the cake batter, and the meringue. Use a spatula to take ⅓ rd of the meringue and add it into the egg yolk and flour mix. Gently fold it so the batter is lightened. Now add in another ⅓rd of the meringue and use the spatula to fold in cut and fold technique. This aerates the batter and lightens it even more. Now add in the last ⅓rd of the batter and fold gently. Now the batter will be super light and fluffy.

Take the prepared cake pan. Pour the batter into the cake pan from a height so there is no air bubbles. Scrape the bowl really well to transfer all the batter into the cake pan. Tap the pan multiple times on the countertop to remove any excess air bubble.

Now place the cake pan into the larger baking pan. Pour the hot water into the larger baking pan carefully so no water splashes over the cake batter. Gently place the cake pan into the lower rack of the over. Cover the oven and bake the cake for 60 to 70 minutes.

Once baked, if you touch the top of the cake it should be firm to touch. The cake will be slightly wobbly. The top will be golden brown in colour.

Remove the cake from oven. Gently remove it from the waterbath. The cake will slightly collapse as it cools. Let it cool in the cake pan for 5 minutes. Remove the cake from the pan with the help of parchment paper. Peel off the parchment paper, let the cake cool completely before slicing and serving.