Now you are asking, Mrrzy. I would probably just use one block of dark cooking chocolate for that amount of cake. To ramp up the chocolatey-ness you could also add a couple of heaped teaspoons of cocoa.

I would mix the cake ingredients together and add the melted chocolate at the end.

I usually melt chocolate on half power in the microwave one minute at a time and stir it after each minute. It usually takes about 3 minutes all up.

I'm tempted to try this out, but using the orange as well. I love orange and chocolate flavours together.

Also, when I boil the oranges I keep the water in a bottle in the fridge and add a bit to cold mineral or soda water, usually with a splash of Angostura bitters. Yum in summer. Very refreshing.