Melt 2 tablespoons unsalted butter in a large pot over medium. Add 1 bunch large leeks (white and pale-green parts only), thinly sliced, washed well, and dried (5 cups). Season with coarse salt and freshly ground black pepper. Cook, stirring occasionally, until leeks are tender, 15 to 17 minutes. Transfer to a bowl; let cool completely before stirring into mixture in place of kale at end of step 3.