Preheat the oven to 350F. Beat the egg yolks, sugar until they turns smooth and plate,add the flour,butter and chocolate powder to them, mix everything well and keep aside.
Now whisk the egg whites with a pinch of salt until soft peeks forms. Fold the whipped egg whites to the already prepared chocolate batter, mix gently until they get well mixed.
Line a baking sheet over a swiss roll pan or a baking tray,spread this batter and bake for 12minutes,until the top turns firm, remove the baking sheet carefully.
Keep aside and let the sponge cake gets cooled. Now spread the already prepared chocolate mousse, roll them, arrange them in fridge for few hours and enjoy them.