Have a look at the little 'Unai's first day - behind the scenes' video.

[Burney]

Yep. 2 egg yolks, some lemon juice and mustard and seasoning, gave that a momentary blitz, then poured the melted butter in. It just hollandaised, instantly. I popped it in a thermos and it held perfectly for half an hour.

You poured the melted butter in while smooshing or poured it in and then smooshed?

These details are key.