Unfortunately, I do not have any cocoa powder left. If I use Cadbury's hot chocolate mix, would it give me the same effect?
You definitely want to use cocoa powder if you can...but...
I've done it in a pinch. It's not as good as cocoa powder though because hot chocolate mix is usually also sugar & sometimes powdered milk. But it's still something to fix a chocolate craving when you don't have much else in the cupboard.
I've added hot chocolate mix to waffle mix/pancake mix and always found it delicious, though of course not as chocolate-y as with pure cocoa powder.
I have used hot chocolate mix instead of cocoa powder... As state earlier it is usually a blend of cocoa, sugar and milk powder... so, you need to adjust the sugar levels. However, also be aware that some of the new style hot chocolate mixes that want to emulate the milky froth of a hot chocolate made with an espresso machine also contain a frothing agent... The "rise" I got from this mix was very "interesting". (and the mess was unwanted)
Another substitute I found for cocoa, is chocolate pudding mix, or even on one occasion a chocolate pudding cup; this produces a very rich, moist chocolate mug cake, or a really nice enhancement for a packet chocolate mix.
I personally would recommend using cocoa powder as it gives it that nice rich chocolaty flavoring.However, if you don't have cocoa powder use hot chocolate mix but keep the sugar down on your original mix as it will become too sweet. Also, the colour of the cake will not be as dark as it would be with cocoa powder