Take 300mls cold milk, preferably in the milk container. Pour on one pack butterscotch Angel Delight. Close bottle. Shake well for half to one minute.
(Alternatively whisk by hand or blender in mixing bowl. Or if you're me using a fork does just as well, just takes longer ...)
Mixture by this time will be well thickening.
Quickly pour out into a bowl. Leave to thicken for a further five minutes, by which time it has the consistency of a light mousse...
... and voilà! Superbe!

PPS my googlings have unearthed that foreigners believe Angel Delight is a British phenomenon. Not so! The local shop does a Turkish version, only the chocolate one's oversweetened and the strawberry is OK, with chocolate bits but it's 89p a pack whereas Morrisons Value strawberry is 6p (and adding a bashed-up Cadbury's Flake is far superior to the Turkish premixed brand... and Angel Delight proper currently retails at 47p... not bad.