Cook the potatoes in salted water until tender and drain through a sieve. Put the pan back on the fire and bring the cream to the boil, add the butter and season with salt and pepper. Leave to reduce until the cream starts to thicken. Add the potatoes and mash the whole into a smooth puree.
Grill the Wagyu briefly over the hot Binchotan. Cut the meat into thin slices and serve with the mashed potatoes.