If you don’t have a special milkshake machine, move the ice cream from the freezer to the fridge 5 minutes or so beforehand so that it is slightly softened and easy to whisk.
Whisk the ice cream in a bowl until it is soft and creamy. You can do this by hand, or with a hand-held blender. Add the milk and whisk until smooth and frothy.
Trickle most of the sauce around a tall glass so that it clings to the side and then pour in the milkshake. Drizzle the last of the sauce on top and serve at once, with straws (everything always tastes delicious with straws).