Whisk the egg yolks and the vinegar briskly in a metal bowl set over a saucepan with about an inch of simmering water at the bottom. If you see even a hint of curdling in the eggs, take the bowl off the heat, but keep whisking. Keep going back and forth from the heat until the mixture turns thick and lightens in color. Whisk in a tablespoon of warm water.
Begin adding the softened butter, a pat at a time. After about a fourth of the butter is in there, you'll begin to see a change in the texture of the sauce. At that point, you can step up the addition of the butter a bit, and keep going till all the butter is incorporated.
Whisk in the cayenne and the lemon juice. Cover the bowl with plastic wrap and set in a bigger bowl of warm (not hot!) water.
Use a large stainless-steel skillet filled with water about an inch and a half deep. Bring it to a boil while dissolving the salt into it and adding the vinegar.
The hard part of poaching eggs is keeping them together as you add them to the pan. The best trick is to use a coffee cup--the kind that narrows at the bottom. Break one egg into each of four cups. (Or eight, if your skillet is big enough to fit all those eggs.)
When the water comes to a boil, lower the heat to the lowest possible setting. Slide the eggs carefully into the pan, two (or four, even) at a time. Let them simmer for three to four minutes, depending on the size of the eggs.
The best tool to remove the eggs with is a round skimmer with holes in it, or a large slotted spoon. Carefully remove one at a time, and let the excess water drip off.
Place a slice of smoked salmon atop each English muffin half, then top with an egg. Cover with a generous flow of hollandaise. Garnish with the fried capers.