I was making a white chocolate topping for a bakewell flapjack. It's my own recipe so I have nothing to follow. I've made it before and the chocolate topping was smooth and poured onto the cooled flapjack no problem. I added some butter to the chocolate to give it a fudgy texture but today when I did this the whole thing just seized. I've ended up putting little blobs of the topping on because I couldn't spread it and so now it has the unfortunate appearance of mashed potato!

I didn't get the chocolate too hot (warmed in short bursts in the microwave - again, I've done this before and never had a problem) then the butter added just as it was almost all melted. Maybe I melted the butter with the chocolate last time, could that be it?

Water and burning will do this. As you said it was not too hot then perhaps water. Although if using a microwave do not nuke for long, best to get it warm and slowly stir it. Butter should not do this but if it was low fat butter it could as that is an emulsion of water.

That is so frustrating.

Yes, very frustrating!

You may have hit the nail on the head actually because the only unsalted butter I had was a spreadable one. Thanks for that, I'll know better next time!