My mother sent me a box of Movie Theater Butter popcorn (Pop Secret) in the mail and I'm down to my last bag! Where can I get it here? Or, if not the same kind, some decent type of microwave popcorn? Orville Redenbacher popcorn would be amazing, too, or Newman's Own.
I don't mind trying some UK brands but the only one I can find here is Butterkist and it's absolutely terrible. I've bought it a few times when I've been desperate and I always regret it. I don't understand why they have one flavour called 'butter' and another called 'salt'... Surely the perfect popcorn tastes of both butter and salt 
I realise now this is one of those things that I got really used to eating in the US and is probably pretty uncommon outside the US. But it's too late - I miss it!
Why wouldn't they have a butter and salt flavour? The perfect popcorn for you is not necessarily the perfect for someone else. I don't like butter on my popcorn, I just like it salty. The butter version is salty as well as buttery, or if it's not salty "enough", it's pretty easy to add salt to something...
The brand I see most often in shops is "Popz" I don't know if that is worth trying, they have a butter flavour, also Ocado sells Jollytime which is an American brand, but I think they only sell a plain flavour.
Personally I just make it in a pan from the kernels, it takes like 2 minutes, you can make it with spray oil to reduce calories
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If you really wanted to, you could melt butter and pour it over, you can also buy those flavour shaker things on amazon.