Add the potatoes, white beans, 4 cups water or unsalted vegetable broth, 1 (about 13-ounce) can full-fat coconut milk, the remaining 2 teaspoons kosher salt, 1/2 teaspoon red pepper flakes if using, and the remaining 1/2 teaspoon black pepper to the pot. Increase the heat to medium high and bring to a simmer. Reduce the heat to medium and cook, stirring occasionally, until the potatoes are very soft, about 35 minutes.