[eqdw]

Make them quote a number first.

Do not tell them what your previous salary was.

Whatever their number is, ask for $15k more

If they are a strong no, try and trade off that $15k for increased benefits in some way

If they are a weak no, lower your ask.

They will almost certainly give you $5k above what they quote, maybe $10k

And keep in mind: if you're a typical software engineer going through a typical full-day on-site software engineering interview, they've dumped the better part of $1000 worth of dev-hours just to interview you (double that if they flew you out!) so seriously, asking for an extra $5k is no skin off their back. Just do it.