Easy, No-Fail Sponge Cake

Finally, I am very happy with this sponge cake recipe (in Vietnamese called " banh bong lan"). I found this recipe in an old community cookbook where elderly people met at churches and brought pies or cakes. Now, I can stop searching for the perfect sponge cake recipe because this is the right and easiest one. Feel free to add flavorful ingredients to this recipe, such as coconut, raisins, etc.
Ingredients:
3/4 cup all-purpose flour
1/4 cup cornstarch
11/2 tsp baking powder
1/2 tsp salt
1 tsp lemon juice
1/4 cup white sugar (for egg whites)
1/4 cup white sugar (for egg yolks)
1 tsp vanilla
1/4 cup melted butter
1/2 cup boiling water
4 eggs, separated
2 tbs soft butter for cake pan

Directions:
Combine flour, salt and baking powder and sift three times.
In a bowl combine melted butter and hot water and set a side
Line an 9 inch square or round cake pan with parchment paper. Spread soft butter to cover the bottom and all the side

Preheat oven to 350°.
In a stand mixer, beat egg white , lemon juice with sugar until stiff (about 5 minutes). Add vanilla but do not stir. 

Beat egg yolk with sugar until light and fluffy. Pour hot water-butter mixture in the thin stream into the egg yolk with flour; continues whisking until all combine.

Lightly  Fold the egg white into the egg yolk.

 Pour batter into the prepare pan and bake for 35 to 40 minutes.