This begrudgingly sickly yet alluring pudding feature enjoyed a window of mid-1980s ubiquity when any mum with a modicum of sense made sure a substantially-filled bottle was always ready in the cupboard. Ice Magic lived in a crappy plastic squirty pyramid at room temperature, but when applied over ice-cream turned from runny liquid to a rock-hard solid, forming – depending on the application – a wafer-thin veneer (for wimps) or a massive crust (for die-hard dessert denizens). The only problem was you had to wait fifteen minutes for the thing to harden, by which time you’d invariably excavated all the ice-cream and were left with a lump of chocolate you may as well have got from out the biscuit tin.

Blue Peter
My family were just a bit miffed when this vanished in the early 1990s.
Recently my girlfriend had some sauce that set in a similar way but was a different brand.
Still going strong here in the US, under the name Magic Shell.
The trick is not to have your ice cream in a single lump, but rather have lots of crevices for the stuff to fill and harden in. More chocolate* that way!
* not actually chocolate, and avoid the other flavours.
I remember Blue Peter did one of their making things specials and they stuck googley eyes to the lids, to make ghosts.