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