Mix the butter, vinegar, mustard, 1 1/2 teaspoons salt, and spices. Line a sheet pan with potatoes. Put half the butter mixture on the potatoes and, using your hands, toss the potatoes in the butter. Alternatively, you can melt half the butter and pour it over, but I would rather dirty my hands than a pot. Sprinkle potatoes with additional 1/2 teaspoon salt.