Remove the outer leaves of the Brussels sprouts and optionally trim the ends of the stalks.
Clean the heads for dirt and the like under cold running water.
Dry them thoroughly.
Put a dab of butter in the bag followed by your Brussels sprouts.
Cook the Brussels sprouts in your water bath at 83 degrees Celsius (181 degrees Fahrenheit). I have tested several different timing options and found that 40 minutes is too little, but 60 minutes is just right.
After the 60 minutes, take the Brussels sprouts out of the bag and serve immediately. If you want to make them a bit extra delicious, you can halve them and give them 30 seconds on a hot skillet, cut side down.