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

[Burney]

Whilst smooshing. I sought assistance from tinternet and watched a video of a bloke doing it. He drizzled the butter in slowly whilst smooshing - I just sort of poured it in and it made no difference.

I've been doing it in a blender like that for some time with considerable success, so that makes sense. However, the problem with the blender is the bloody mess. this sounds better.