Melt the butter until boiling, boil for some seconds & set aside. Attention: it'll bubble along ...
Add egg yolks, lime juice, mustard, yoghurt & sugar to a small & high pot & blend w/ your handheld blender.
Add the liquid hot butter while continuing blending & blend until thick.
Add vinegar (if you like) & salt & pepper.
Add some drops of water if you think the sauce is too viscous ...
Serve at once or store in your fridge ...