Kue Bolu Coklat (Chocolate Sponge Cake)

I remember once my late grandmother making this cake when I was young once and started to like making something simple in the kitchen. There were currently no electricity mikser that time, so my grandmother only use manual eggbeater that looks like a circular and supple pear, but strong. Too bad we do not keep it anymore. "Kue Bolu" is just we always call. 

Now with the various developments and culinary techniques this sponge cake now beginning to develop into a modern recipe with the addition of several 'emulfier' and other additional materials sponge cake is more popular and varied from simple old recipes. 

Trying again this old recipe without emulfier, this time I make chocolate version because my first intention is going to make tiramisu later from this basic sponge cake

Ingredients:
For pan size of 22 cm round.
-1/2 cup flour
- 1 +1/2 tablespoons cocoa powder
- 5 egg yolks
- 4 egg whites
- 1/2 cup caster sugar
- 100 gr unsalted butter. Melted.

Method:
1) Pre-heat oven temperature of 160 deg C. Grease pan with butter and sprinkle with small amount of flour. Mix and sifted flour and cocoa powder. Set aside.
2) Boil the water over a large pan, combine eggs and sugar in other container (smaller than boiling water pan). Beat eggs until fluffy over boiling water (double boil method). After eggs fluffy and pale, remove from double boil and continue to whisk until room temperature.
2) Fold in the flour mixture and cocoa powder, stirring with a spatula until completely mixed.
3) Combine melted butter into it and fold again until batter is shiny and butter mixed well.
4) Pour batter in baking pan and bake in oven until done (30 ~ 35 minutes). Check with the tooth pick, if inserted in and comes out clean, cake is done.

Note: Know the condition of your oven. Different size and condition will give different  heat result.