Powdered Cadbury's Whisper.
I like to get Whispers, freeze them until they're hard, then either grate them with a cheese grater or smash them with a hammer until they're reduced to powder and small chunks.
It's much nicer than any generic kind of desert toppings and goes really well in yoghurt too, plus the greater surface area of the powdered chocolate bar makes it tastier than just eating the bar normally.
However, I don't restrict myself to just Whispers (although it is my favourite), I'll use any chocolate bar that takes my fancy... basically if I can freeze and then smash it, I'll put it on ice cream or in yoghurt.