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.