Halve the heads of baby bok choy lengthwise, then cover them with cold water in a large bowl or dishpan. Swish the greens around vigorously to dislodge any dirt from between the leaves -- it will sink to the bottom of the bowl. Dry the cleaned bok choy in a salad spinner or on a kitchen towel.
In a small skillet, toast the sesame seeds over medium-low heat, until they have turned a light golden brown. Set aside.
In a large (12-inch) skillet, heat the butter over medium-high heat. When the butter stops foaming (but has not begun to brown), add the heads of bok choy, cut side down, and do not move them for about three minutes.
Drizzle the soy sauce and water over all of the bok choy, getting it into the nooks and crannies of the leaves. Use a pair of tongs to turn the bok choi over so it's facing cut side down, then cover the skillet for three minutes.
Remove the lid from the skillet and jostle the bok choy a bit with the tongs. Cook until most of the liquid has evaporated and formed a light glaze. Transfer to a serving plate, sprinkle with sesame seeds, and serve.