You have attempted one of the most precise baking exercises of all... The Victorian sponge. I personally prefer to use Delia Smith's recipe. It make for a super light sponge. To make it "right" ensure that your butter and your eggs are at room temperature. My ingredients (pardon my metric) are as follows...
- 225 g Butter (8 oz)
- 225 g Caster Sugar (8 oz)
- 4 x Medium Eggs
- 175 g Self-Raising Flour (6 oz) Sieved
- 50 g Cocoa Powder (2 oz)
If I deviate from this I am sure to end up with pancakes... Make sure that you sieve your flour well... It needs lots of air.
Chocolate cake on the other hand is a completely different monster.
- 150 g self-raising flour, sifted
- 50 g cocoa, sifted
- 220 g caster sugar
- 80 g butter, softened
- 125 ml milk
- 2 x eggs lightly beaten
Its a much heavier recipe; less SR Flour, more mechanical integration (4 minutes instead of one; more moisture in the form of milk; less eggs...