Cook potatoes in a large saucepan of boiling salted water until tender. Drain and transfer into a bowl and mash with a potato masher.
While the potatoes are still hot, mash them with the crème fraîche, egg yolk, mustard, sea salt and freshly ground black pepper. Mix in the scallions, leeks or cabbage.
Add 1 tablespoon of flour to the potato mixture to help make the consistency suitable for rolling out and sprinkle the remaining tablespoon of flour on the work surface. Roll the potato mixture out to 5 centimeters thick using a rolling pin and cut into circles using a 5-centimeter scone or biscuit cutter.
Melt the butter in a frying pan and fry the potato cakes gently on both sides until golden-brown.