Fill a saucepan with at least 4 cups of water and bring to a boil over high heat. Meanwhile, prep the celery root. First, cut off the bottom, so the root is upright and stable. Next, cut away the fibrous outer skin, following the shape of the root. Then cut into even ½-inch cubes. Add to the water as it heats up, sprinkle liberally with salt, and add butter. When the water comes to a boil, reduce heat to medium and simmer 30 minutes, uncovered.
Check the celery root for doneness; it's finished when the cubes are tender. Remove celery root with a slotted spoon and place in a blender along with harissa paste and ¼ cup cooking water. Blend on high to purée, 30 seconds. Taste to adjust seasoning, if necessary.
Place purée in a bowl, garnish with chives, and serve.