Sugar syrup blended with powdered milk and a spoonful of butter. Ten minutes, four ingredients, and it makes about a litre, which is roughly three tins. It thickens considerably as it cools.
This makes about a litre, roughly three tins' worth, and it keeps two weeks in the fridge in a sealed jar. Add the powdered milk gradually with the blender running: dumping it in at once gives lumps that never break down. Whole milk powder gives a result much closer to the tinned version than skimmed does. It will look alarmingly thin coming out of the blender and it is meant to; the thickening happens in the fridge over a few hours. If it is still too loose the next day, that is the point to blend in more powdered milk, not to cook it further.