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Thanks for your concern. We've frozen it for now, but we're thinking, first of all, of getting the vet to send it for testing before we do anything. Luckily the dogs seemed to loose interest in it and are all fine - the little one won't have anything to do with big chunks of anything, the biggest is a finicky eater and wouldn't touch it, and the middle one, my own dog, left it alone. That's what made us think something wasn't right. She normally loves chocolate (and no, she doesn't get any as a rule) and would have picked up, taken it into a corner and guarded it - but she didn't.
It rather looks as though someone's used cooking chocolate, melted it down and remoulded it into something like an ice-cube tray. It's very crudely done, as I said, and doesn't look like commercially moulded stuff at all.
There are circumstances that lead us to think someone could be out to poison the dogs, so we're keen to get this sorted.