I've made one, but it didn't rise very well ( I do cakes as a hobby & to sell to friends) - when I do a vic sponge it's to die for - very light & rises well. My choc cake is just the VS recipe substituting some flour for cocoa powder. I don't get much call for choc cakes, as most people tend to prefer the VS, so I have had more experience making these as opposed to the choc (only started couple yrs ago). Is it the cocoa powder that weighs it down? I follow exactly the same rules for my VS & add baking powder?
I cream the butter & sugar in a mixer. Then I mix the flour & cocoa powder & sieve twice, then add gradually to the mixture along with eggs. I put it in the middle of the oven on 180 when the oven is warm - not hot, for approx 25 mins. It's exactly the same as I do for the VS, except with the cocoa powder.