First make the Hollandaise sauce.
Put the lemon juice and vinegar in a small bowl, add the egg yolks and whisk with a balloon whisk until light and frothy.
Place the bowl over a pan of simmering water and whisk until mixture thickens.
Gradually add the butter, whisking constantly until thick – if it looks like it might be splitting, then whisk off the heat for a few mins.
Season and keep warm.
To poach the eggs, bring a large pan of water to the boil and add the vinegar.
Lower the heat so that the water is simmering gently.
Stir the water so you have a slight whirlpool, then slide in the eggs one by one.
Cook each for about 4 mins, then remove with a slotted spoon.
Lightly toast and butter the muffins, then put a couple of slices of salmon on each half.
Top each with an egg, spoon over some Hollandaise and garnish with chopped chives.