Failed chocolate sponge
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 :)