Ok, cut the chestnuts cleanly across the top or rounded side, not the flat side.
Put them in the tumbler glass and fill it just to the top of the resting chestnuts.
Put the glass in the microwave and put on high. Now watch carefully for heavy boiling of the water. Next make sure you see bubbles coming out of the crosses on the chestnuts. This indicates they are steaming. The water replaces the liquid in the chestnut and keeps them moist. The power of the oven and the mass of the chestnuts change the amount of cooking time, but should only be a couple of minutes. Now here is the trick. Carefully drain the water off and set the glass with the chestnuts in the freezer for 10 minutes.
Why? because it make them easier to handle and the shells are soft and easy to peel off the nut. With a little practice, you can cook, cool, peel and be ahead of those eating the hot chestnuts. Enjoy.