If Darth Vader built C3PO, why did he never recognize him?
Memory wiped at the end of episode 3.
His memory was wiped at the end of revenge of the sith
To be fair, the old Expanded Universe elaborated on that bit: He had C-3PO melted down because of being forcibly reintroduced to his bad past, but after Chewie saved him, Vader, in a surprisingly nice moment for him, ultimately allowed Chewie to repair him (something that was implied in the film when C-3PO’s remains were supplied to Chewie from what was essentially a doggie door).
Don’t know about the Disney EU, though. Apparently, the SCAR comic didn’t have Vader recognizing C-3PO at all despite the titular unit interrogating him [and succeeding to such an extent that they actually wanted him to shut up, though I haven’t fully read that comic, to be honest so I might be mistaken.].
Actually, in the comics he did. Or so I’ve heard.
Might have been a novelization.