3 pounds russet potatoes, peeled and cut into 1-inch cubes (see note)
2 tablespoons grated fresh wasabi (grate it 5 minutes before combining with milk mixture)
1/2 cup unsalted butter, melted (1 stick)
1 1/4 cups half-and-half or light cream (use whole milk for less richness)
1 teaspoon freshly ground white pepper (you can use black pepper, but it leaves flecks in the white potatoes)
1 teaspoon salt, or to taste
Bring the potatoes to boil in a large covered pot of cold salted water. Lower the heat and boil until tender when pierced with a fork, about 12 to 20 minutes.
While the potatoes are cooking, in a small saucepan, melt the butter in the milk (use moderately low heat). After the wasabi has stood for 5 minutes after grating, stir it into the butter and milk mixture.
Once the potatoes are cooked, drain them in a colander. Return them immediately to the hot pot and shake them over high heat for about 30 seconds to dry them out.
Peel and mash the potatoes (a ricer is the most efficient way, or use an electric mixer, taking care not to over-blend) and stir in the wasabi cream, salt and pepper.
Note: You can leave the potato peel on for a rustic presentation.