Slice off any hard ends of the Brussels sprouts, as needed. Slice them in half lengthwise.
Place about 1 inch of water in a pot or saucepan. If using a steamer basket without a handle, add it to the pot now: the water surface should be right under the basket. Bring the water to a boil.
Once boiling, add the Brussels sprouts (in the steamer basket, if it has a handle). Steam with the lid on until just tender when pierced with a fork, about 6 to 7 minutes. The timing is important: you’ll want it just tender all the way through: not too crisp, but before it starts to loose its bright green color.
Carefully remove the Brussels sprouts to a bowl. Toss with the butter, kosher salt, chives and fresh ground black pepper. Mix until the butter melts. Serve immediately. Store leftovers refrigerated for up to 2 days.