Begin by placing the potatoes in a large saucepan of water and bringing them to the boil. Simmer for 20 minutes or until the potatoes are tender. Strain the potatoes off the water, mash with 50 grams butter and season with salt and pepper. Leave aside to cool for ten minutes and add the egg yolk and the flour. Combine thoroughly and shape four patties of five centimeter diameter and two centimeter thickness.
Heat the oil in a pan over a medium to low heat, add the potatoes cakes and cook for five to eight minutes until golden brown on both sides. Keep warm in the pan until ready to serve.
Meanwhile, prepare the poached eggs. Fill a saucepan with water and add the vinegar.
Bring to the boil over medium-high heat until simmering with small bubbles rising from the base of pan and small ripples across the top of the water.
Using a wooden spoon or whisk, stir simmering water in one direction to create a whirlpool (this will help to give your poached eggs a neat shape).
Slide egg from saucer into the center of whirlpool, as close to water as possible. Cook for two to three minutes for a semi-soft yolk without stirring. Do two or three eggs at the time if desire.
Using a slotted spoon, remove and drain on a plate lined with paper towels.
To serve, transfer each potato cakes onto serving dishes. Add smoked salmon slices, and top with the poached egg. Drizzle some hollandaise sauce and serve immediately.