Preheat your oven to 320°F (160°C) and line a 6-inch square cake tin with parchment paper. Make sure all your ingredients are at room temperature.
Add egg yolks + 40 gms to sugar to a mixing bowl and beat for a minute. Add oil, vanilla extract, salt, milk, water, lemon juice and zest to the bowl and mix in till everything gets combined well.
Now add in at least three times sifted cake flour and baking powder to the mixture and fold it in.
In a separate clean and dry bowl, add egg whites (at room temperature only) and beat in for a minute. Now add cream of tartar/lemon juice to it and beat well. Slowly keep on adding the remaining sugar and beating continuously until the egg whites form stiff peaks. This may take 5-6 minutes.
Mix egg whites in the yellow cake batter in three batches. And fold in using a spatula. Then pour the mix in your lined cake tin and bake for 25-30 mins or until a tooth pick inserted in the center comes out clean.
Invert the cake tin, as soon as you take it out of the oven and let it cool down completely.