Failed chocolate sponge I'm new to bakery and I can bake chiffon and angel food cake alright, but everytime(3 times) I bake chocolate sponge, I always end up with something with good moisture but super stiff. What could I have done wrong and how can I make the cake more tender in texture? Thanks!

My recipe: 3 eggs, 40g butter, 40g(in egg white)+20g(in yolk) sugar, 60g flour, 20g cocoa powder for a 8-inch square baking pan. 170°C, 10min.

And what I did:

Separate the egg white and yolk, whip the white until there's a curly spike when I pick up the wisk. During the whipping process, pour in the 40g of sugar. 

Add 20g sugar to the yolk and stir until the mixture gets a bit pale. 

Add 40g of melted butter into the mixture and stir until the mixture is smooth. Pour 1/3 of the egg white foam into the yolk-butter mixture. Mix carefully as not to deflate the egg white. When mixed well, pour in the other 2/3 of egg white and mix. 

Add in the flour &amp; cocoa powder. Bake in preheated oven.
Sponge and pound cakes contain fat. Therefore, they are not usually made with pure egg leavening. Egg leavening relies on a protein-based foam (whipped eggwhites), and fat deflates protein-based foams and/or prevents them from "curing" at high temperatures. 

If you found a sponge cake recipe without chemical leavening, this is a sign for a bad recipe. The best would be to search for a better recipe, especially if you are a beginner baker. Troubleshooting baking recipes is an advanced skill which requires both experience and theoretical knowledge - certainly attainable for the average home baker, but unnecessarily hard for beginners. With the tons of recipes available at your fingertips for free, it makes more sense to search for a better one. It will give you correct ratios and temperatures and save you failures and frustration. 

And a small pedantic part: what you are making here is not a sponge cake, but a pound cake. They have the same ingredients, but sponge cakes are made using the creaming method, while pound cakes use melted butter. The upside: both of them are tasty :)