Bring a pot of salted water to a vigorous simmer over high heat. Turn the heat to medium, add the potatoes, and cook until just tender, 14 to 16 minutes. Drain the potatoes and transfer to a medium bowl. When the potatoes are cool enough to handle, add 2 tablespoons of the butter and the mustard and smash with a fork, preserving a chunky texture. Season with vinegar and salt.