Bring poaching water to boil and add the vinegar. Lower the heat to a simmer.
To make the hollandaise sauce, melt the butter. Put egg yolks, lemon juice and salt in a blender, and blend together. Slowly add the melted butter to the egg yolk mixture. Set aside.
To poach the eggs, crack an egg into a small bowl and slip it into the simmering water. Turn off the heat and let sit for 4 minutes. Remove the egg and follow the same step for another egg.
Toast the bread buns and cut in half. Put a poached egg on top of the bun, then pour some hollandaise over. Sprinkle some spring onions over it and serve.