Bring a stockpot of water to a boil and salt it. Meanwhile, trim the greens and rinse them well. If you’re using sturdy greens, separate the leaves from the stems and chop both separately. If you’re using tender greens, roughly chop any big leaves.
To cook sturdy greens, first add only the stems to the boiling water; when the stems are almost tender, 3 to 4 minutes later, add the leaves. To cook tender greens, add everything to the boiling water all at once. Cook until the leaves are just becoming bright green and tender: 1 to 3 minutes for watercress and arugula, 3 to 5 minutes for spinach, 5 to 7 minutes for chard, and 7 to 10 minutes for kale, collards, and bok choy.
Drain the greens in a colander or strainer, pressing down lightly with a large spoon to remove as much excess water as possible. (Or “shock” them in a bowl of ice water to stop the cooking, then drain and squeeze them dry with your hands.) Transfer the vegetables to a large bowl and toss them with the butter and a sprinkle of salt and pepper. Taste, adjust the seasoning, and serve right away.