A plea for anyone having this happen, be gentle.
It is easy to think a tantrum is because the child wants something and either gets pissed that they don't, or are trying to make a scene to get it.
Sometimes (often?) it is more of a stress and panic reaction. And it can be hard to tell. Even if the tipping point was not getting biscuits.
Whether your reaction is to give in and buy them something, remain very firm that they do not get biscuits today and this is not acceptable behavior, or decide to pick them up and carry them to the car, be a safe and calm presence for them.