2) Meanwhile, preheat a large frying pan or skillet over a medium-high heat and add 30g of the butter. Season the salmon all over with salt and pepper. When the butter foaming subsides, place the salmon in the pan and cook, turning once, until well-browned, about 1.5 minutes per side. Transfer to a baking sheet.
3) Put the cooked potatoes in a small bowl with the 2 tsp of salt, 30g of the butter, the single cream and the horseradish and, using a fork, mash until fluffy. Season with pepper, to taste.
4) Melt the remaining 30g of butter in a small saucepan. Using your hands, press about 1/4 of the potato mixture onto the top of each salmon fillet, to form a thin crust. Lightly brush the potato with the melted butter.
5) Place the salmon under the grill and cook until still slightly pinkish inside and the crusts are golden brown, about 10 minutes. Transfer to a serving platter and garnish with the chive segments. Serve immediately.