Is Weisst also correct or should it be Weißt?
Yes, it’s also correct. You can virtually always write “ss” instead of “ß” (but not the other way round). Everyone will still understand you. It’s essentially the same, just an alternative look. It’s not an entirely different letter as with o and ö, for example.
Over the past 30 years, Germans tried multiple times to regulate how words are supposed to be spelled, often reversing the previous decisions, which got everyone confused and 10 year old children were learning different spellings that their parents did when they were young, so nowadays both spellings are accepted.
The rule I learned as a kid: when there are two vocals next to each other, like “ei” or “au”, you write “ß”, not “ss”.
For “wissen”, there is no correct alternative spelling. It has to be “ss”.
But for “Weißt du …?”, since there are two vocals next to each other, technically the correct spelling is with “ß”, at least when I learned it. But nobody will go mad if you write “Weisst du …?” It might even be that my knowledge is outdated, but it doesn’t really matter anyway. Especially since there is auto-correct 
There are some words where you need to be careful, like:
- “weiße” as in “weiße Farbe”, which means “white color”
- “weise” as in “der weise Mann”, which means “the wise man”
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And some German-speaking regions have done away with ß altogether, like Switzerland. They just write “ss” instead.
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