Direction :

1. Place the potatoes and the 2 teaspoons of the salt in a medium saucepan, add enough water to cover, and bring to a boil over high heat. Reduce the heat to moderate and simmer until the potatoes are tender when pierced with a fork, about 45 minutes. Drain, reserving at least 1/4 cup of the cooking water — I like to reserve at least a cup of the cooking liquid. 
2. Return the potatoes to the pan and set over low heat, uncovered, for about 5 minutes, stirring occasionally, to let the potatoes dry out a little. (See notes above if using a ricer or food mill.) Add the buttermilk, ¼ cup of the reserved cooking liquid, pepper to taste, and a pinch of sea salt. Mash the potatoes with a potato masher until you have a coarse purée. Add the butter and mash again. Once the potatoes are as mashed up as possible you can switch to a wooden spoon or spatula and beat them further. Taste. Add more sea salt and pepper to taste. Thin with more reserved cooking liquid if desired.
3. To make the herby sizzle: Heat the oil, scallions, crushed red pepper flakes, and cilantro in a small skillet or saucepan over medium heat. Cook, swirling occasionally, until the scallions and red pepper flakes start to visually and audibly sizzle. Season with a pinch of sea salt and remove from the heat.
4. To serve: Transfer half of the potatoes to a serving dish. Use the back of a spoon to swoosh the potatoes around a bit, creating peaks and valleys. Spoon the herby oil over top, allowing the oil to pool in the crevices. Crack more pepper over top and, if you wish, season with another pinch of sea salt.