For the muffins, arrange the muffins cut side up on a baking sheet. Spread the cut sides with butter. Broil until light brown.
For the spinach, sauté the spinach in the butter in a skillet until spinach is wilted.
For the eggs, add just enough water to a saucepan to measure 2 inches. Bring to a simmer. Stir in the vinegar. Break the eggs 1 at a time gently into the water mixture; break the eggs as near the water as possible. Simmer for 3 minutes for a soft yolk or for up to 5 minutes for a firm yolk.
For the sauce, whisk the egg yolks and lemon juice in a double boiler until blended. Cook over hot water until slightly thickened, whisking constantly. Remove from heat. Add the melted butter to the egg yolk mixture gradually, whisking constantly until blended. Stir in the salt and cayenne pepper.
To assemble, place 1 muffin half toasted side up on each 8 serving plates. Layer each muffin half with 1 tablespoon of the spinach, 1 poached egg, 1 ounce of the smoked salmon and 2 tablespoons of the hollandaise sauce. Serve immediately.