My favourite indulgence is Diet Jelly:
Cottees Diet Jelly seems to be the lowest - 125ml for 24.1 KJs
Aeroplane Lite Jelly is 125ml for 38 KJs
I've put it in little jelly moulds (like you use for kids) and can indulge every night for few KJs.
In fact you could eat the whole 500ml for around 100 KJs - reckon that would have to be the lowest KJ for a dessert ever!