Preheat the oven to 180°C.
Layer potatoes in an ovenproof dish, pour over melted butter, and bake for 15-20 minutes until tender.
Mix shrimps, tarragon, and enough mayonnaise to bind the ingredients, then season with salt and pepper.
Dress the watercress leaves with olive oil, salt, and pepper in a large bowl.
Plate a spoonful of potatoes, top with shrimp salad, create a central indentation, and add watercress.
Heat a pan of water to 70°C and gently poach the egg yolks until cooked to your liking.
Place one poached egg yolk into each shrimp salad indentation and serve with warm muffins.