Method

To make the hollandaise sauce, place the white wine vinegar, black peppercorns and tarragon sprigs (if using) in a small saucepan. Bring to the boil and reduce down by around half, then strain into a jug.
Place the butter in a saucepan and melt until the it has separated and the solids are at the bottom of the pan, skimming off any residue from the top with a spoon.
Place a small saucepan half filled with water on a low heat, with a heat proof bowl securely on top. Add the yolks to the bowl with vinegar and whisk well.
Continue whisking and very slowly incorporate the golden liquid butter by gradually dripping it into the warming bowl, until you have a bright, smooth hollandaise sauce. Add a little lemon juice and keep the bowl on top of the pan of water to stay warm whilst you poach the eggs. Whisk the sauce occasionally to stop it from splitting, then turn off the heat.
In a frying pan, lightly wilt the spinach in a little butter.
Place a large saucepan of water on a medium heat. Poach the eggs to your liking by cracking them into the slowly bubbling water and letting them bob around for 3-4 minutes until cooked, then scoop them out with a slotted spoon.
Halve and toast the muffins then butter them. Divide the muffins between the plates. Top with spinach, a sliver of prosciutto and a perfectly poached egg, then spoon over the silky hollandaise sauce.