Yup, trust Jaggy to know what goes into a macaroon bar: mashed potato. Cold, mashed potato mixed with a shedload of icing sugar. It shouldn't work, and yet it does. As evidenced by the lack of macaroon bars left in my house.
An honourable mention goes to Daysgoby for suggesting condensed milk because, as she quite rightly notes, you tend to find that in all really sweet things (and therefore by definition, in lots of things my Husband likes). And a sympathetic pat on the shoulder and an offer of a nice sit down goes to XUP whose outrage was almost palpable. Sorry for upsetting you so much, XUP, but you made the cardinal mistake of confusing macaroons (nice, coconut-y, melty biscuits) with macaroon bars which, as you now know, are solid bars of sugar-laced mashed tattie.
Actually, reading that back, you shouldn't be surprised that the Scots have invented a dish that combines potatoes and sugar - it was only a matter of time, really. (And on that front, and in retaliation, what the hell is North America doing putting marshmallows on top of potatoes at Thanksgiving? How is that worse than macaroon bars??? I rest my case.)
Anyway, Jaggy wins the Scottish Smartass of the Month award. Not, I suspect, for the first (or last) time.
PS: It has also just occurred to me that since potatoes are vegetables, a macaroon bar can totally count as one of your five a day! In Scotland at least. Score!