Prepare a 6'X6' cake tin, line it with parchment paper. Brush it with melted butter, sprinkle little flour tap on all sides to remove excess flour.Set aside.
Separate 2 egg whites and egg yolk, set aside.
To a bowl add 1/2 cup + 1 teaspoon flour along with 1/2 teaspoon baking powder and 1/4 teaspoon baking soda. Sieve it well then give a quick mix and set aside.
To another mixing bowl first add 2 egg whites along with 1/2 teaspoon vinegar.
Beat it well until frothy at least for 2-3 minutes. Now add 1/2 cup powdered sugar in three parts.
Add little the beat well to combine then add little more and repeat to finish.
Beat until stiff peaks form. It should be stiff so that when you invert the bowl it should not fall off this is the perfect consistency.
Add 2 egg yolks.
Add in sieved flour mixture.
Add 2 tablespoon milk, 2 tablespoon oil and 1 teaspoon vanilla essence.
Gently fold in using cut and fold method.
Fold in gently until no dry flour is seen.
Transfer to the prepared cake tin. Tap gently to remove air bubbles if any.
Bake in preheated oven at 180 DEG C for 25-30 minutes or until a tooth pick inserted in the center comes out clean.
Cool down the cake completely.
Slice them and enjoy!
Homemade sponge cake is ready!