Heat lemon juice and vinegar in a small saucepan just until the mixture starts to bubble and simmer. Meanwhile, place the egg yolks in the small bowl of your food processor (or in blender) and season with a smidgen of salt and pepper. Blend for one minute. Once the lemon juice/vinegar has come to a simmer turn your processor or blender back on. VERY slowly add the hot liquid in a small stream to the egg yolks while the food processor of blender is running.
Using the same saucepan, melt butter over a VERY gentle heat. Do not allow butter to turn brown. When butter foams, turn the processor or blender back on and pour in the butter in a thin, SLOW, steady trickle. The slower you add it the better. Clean the bowl with a spatula and add to the sauce and pulse once.
The key to a perfect poached egg is to keep the water at a bare simmer throughout the cooking. Place a small frying pan over gentle heat and add enough boiling water to fill it to 1 inch. Keep the heat gentle.
Once you see tiny bubbles on the bottom of the pan, carefully break the eggs, one at a time, into the water and let them barely simmer, without covering, for exactly 1 minute. Use a timer.
After one minute, remove the pan from the heat and let the eggs sit in the hot water and set timer for 10 minutes. This timing will give perfect results for a beautifully translucent, perfectly set white and a soft, creamy yolk.
Remove eggs, one at a time, with slotted spoon and let the spoon rest for a few seconds on a sheet of paper towel to absorb the excess water.
When the pancetta is cooked, keep it on a warm plate while you lightly toast the split muffins on both sides. Butter the muffins and place them on the baking tray, then top each half with two slices of pancetta. Put a poached egg on top of each muffin half and spoon over the hollandaise, covering the egg (there should be a little over 1 tablespoon of sauce for each egg).
Flash the Eggs Benedict under the grill for just 25-30 seconds, as close to the heat as possible, but don't take your eyes off them – they need to be tinged golden and no more. This should just glaze the surface of the hollandaise.
Serve immediately on hot plates.