I know it is normally Daleks, but I have seen it writen Darleks too.
I also noticed that the actors too, usually pronounce it "Darleks".
So, are the two spellings interchangeable?
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Here's the most official page I could find about them, the BBC website's character page on the Daleks. If you look them up on the internet, they're ALWAYS spelled "Daleks": on Wikipedia, the BBC website, the Chakoteya episode transcripts, TARDIS Wikia, everywhere. Anywhere you might have seen them spelled "Darlek" is either a misspelling or a reference to such a misspelling (e.g. at Urban Dictionary and the Free Dictionary).
However, yes, the actors and everyone else do pronounce the word in the same way as "Darlek" would be pronounced in a British English accent, i.e. as /ˈdɑːlɛk/ (as opposed to e.g. /ˈdeɪlɛk/ or /ˈdælɛk/). That's just a peculiarity of the English language - specifically, non-rhotic accents of English - and doesn't at all mean the word is spelled Darlek.
There's already a great answer here, but I thought I'd add that it's also definitely "Daleks" because, in the original series episode where the Doctor travels in time to when the mechanical Daleks were created ("Genesis of the Daleks"), we learn that they are the opponents of the "Kaleds", whose name is an anagram of theirs (as if to underline the pointlessness of their conflict).
In fact, Doctor Who himself spells out the name K A L E D S and notes that it's an anagram of Daleks in this scene.
It's always written Dalek, and I've never heard it pronounced with an "r" sound. Perhaps a non-rhotic speaker would, upon hearing the word as I've always heard it pronounced, might infer a "r", especially if they hear it spoken by another non-rhotic speaker (such as Matt Smith).
Of course it's Dalek. The actors in the show itself call them Daleks, Davros calls them Daleks, and if it was Darleks, then I guess the people that invented them would be the Kalreds? No, no no no. It's Dalek, and don't let anyone say otherwise. In fact, if you see someone that says "Darlek" make them wright "Dalek has no R" one hundred times on a screechy chalkboard.
P.S. every time I tried to type Darlek it wanted to autocorrect me, but did nothing when I typed Dalek.