To make the hollandaise, sit a large bowl over a pan of hot water and whisk the egg yolks with 2 tbsp hot water.
Gradually add the melted lactose free butter (without adding the milky liquid at the bottom) until it has all been incorporated.
Whisk in the lemon juice and season with the cayenne pepper and salt to taste, then set aside.
Heat a shallow pan of water with the vinegar but no salt until gently boiling, then poach the eggs for 2 mins.
Remove with a slotted spoon and cool in cold water. Fry the spinach in a wok with a knob of lactose free butter until wilted, then drain and season.
To serve, heat the grill to high. Lay the muffins on a flat roasting tray, cut-side up, then toast until brown. Butter the gluten free muffins lightly, then top each with a ruffle of smoked salmon.
Divide the spinach between the muffins, leaving a slight dip in the middle to sit the eggs in. Spoon a quarter of the sauce over each egg, then return the tray under the grill to brown for a min. Serve immediately.