Another Sponge Cake

I found this recipe in an old recipe book published from a church. It seems like grandma's baked pie and cake that was brought to the church on Sundays for social events. Even with modern equipment, I have to admit that Grandma knew best. This cake comes out so moist and fluffy. Maybe the boiling water is the key?

Ingredients:
1 1/4 cups flour
1 1/4 tsp baking powder
1/2 tsp salt
3/4 cup sugar
4 eggs separated
1 tsp vanilla
2/3 cup boiling water
1 cup coconut(optional)
Directions:
Sift flour, baking powder, and salt three times.
Divide sugar equally into two portions.
Separate egg yolks from the egg white and put in two bowls.
Use an electric mixer to beat the egg whites while adding sugar until thick. The egg white can stand peaked when you lift the beater. Add vanilla.

Use the same mixer to beat the egg yolks while adding sugar until pale yellow and thick. Add a thin stream of boiling water into egg yolks while slowly and constantly stirring so that the yolk will not cook. Sift the flour into the egg yolks mixture. Mix until combined. Make sure there is no lump left at the bottom.

Fold the egg yolks mixture into egg white mixture making sure all the ingredients are incorporated, but do not overmix. Pour the batter into an ungreased baking pan and bake at 350 ° for 50 minutes or until a toothpick inserted into the center comes out clean and dry.