Melt the butter and skim off the white foam.
Whisk the egg yolks and vinegar in a glass bowl, set it above (but not touching) a pan of simmering water and whisk until the mixture thickens.
Take the bowl off the heat and gradually whisk in the buter a knob at a time, until the hollandaise is thick and creamy.
Season with a squeeze of lemon juice, some salt and pepper and add a splash of water to make a spoonable consistency.
Stir in most of the tarragon and spoon this over the salmon in the tart.
Put any leftover into a jug to serve on the side. Finish with the remaining tarragon and capers.