Peel the sweet potatoes and grate them on the large holes of a box grater into a large bowl. Coarse shreds only, not fine.

Toss the shreds with the kosher salt and leave them 10 minutes.

Pile the shreds into a clean kitchen towel, gather the corners, and twist hard over the sink until the liquid stops running. Return them to a dry bowl and fluff them apart.

Add the beaten egg, cornflour, garlic powder, and black pepper and fold until every strand is lightly coated.

Divide the mixture into six and press each portion into a flat round about 3 ½ inches across and ⅓ inch thick, letting a few strands stick out at the edges.

Heat half the butter with half the oil in a heavy skillet over medium heat until the butter foams and the foam begins to subside.

Lay in three rosti, press each once with a spatula, and leave them undisturbed for 4 to 5 minutes until the underside is deeply browned and lacy at the edges.

Flip once and cook 4 more minutes. Wipe the pan, then repeat with the remaining butter and oil and the last three rosti.

Move the rosti to a wire rack, not paper towels, and salt them immediately.

Serve hot, topped with a fried egg for breakfast, or with creme fraiche and sliced chives alongside.