Cut the red potatoes into 1-inch chunks, leaving the skin on for texture and nutrients. Thinly slice the scallions, separating the white and light green parts from the darker green tops (we'll use these separately for better flavor). Chop the cilantro if using. Measure out the gochujang, rice vinegar, soy sauce, soy milk, grapeseed oil, and sesame oil into small bowls for easy access. Bring a large pot of salted water to a boil, then add the potato chunks. Simmer for about 20 minutes until the potatoes are very tender and easily pierced with a fork.

Drain the cooked potatoes thoroughly in a colander, reserving about 1/4 cup of the starchy cooking water. Return the drained potatoes to the hot pot off the heat. Begin mashing them with a potato masher, leaving some texture—we want some small chunks remaining for an authentic Korean-style consistency. I prefer not to over-mash because it creates a gluey texture; a few small lumps add character to the dish.

While the potatoes are still warm, add the gochujang, rice vinegar, soy sauce, grapeseed oil, sesame oil, and soy milk to the pot. Stir vigorously to combine, using the heat of the potatoes to help blend the gochujang smoothly into the mixture. If the consistency feels too thick, add some of the reserved cooking water a tablespoon at a time until you reach your desired texture—creamy but still slightly chunky. The sesame oil adds a wonderful nutty depth that's characteristic of Korean cuisine, so don't skip it even though it's just a teaspoon.

Transfer the mashed potatoes to a serving bowl or individual portions. Top generously with toasted sesame seeds for crunch and visual appeal, then scatter the reserved darker green scallion tops and fresh cilantro over the top. The contrast of the warm, spiced potatoes with these fresh, bright garnishes is essential to the dish's final flavor profile.