Easiest int heworld..I promise. Forget all these others...
150ml double cream
150g Dark cooking choc
Both in a bowl, over a pan of boiling water until melted, mixing frequently.
Put in fridge to chill, mix frequently (every 5-10mins) until consistency of butter.
Spread on cake.
The above quantitiyes are enough for a generous coating in the middle & top of a 8" cake.