Afterpay co-founder Nick Molnar is behind a targeted push inside US payments giant Block to embed the buy now, pay later technology in Block’s Cash App as part of Block chairman Jack Dorsey’s threat to upend the way a generation of Americans bank.

Block shares rallied 17 per cent on the ASX on Friday after Mr Dorsey described buy now, pay later as central to its strategy to lure banking customers to Cash App, used by more than 50 million people in the US.