A treat from What Einstein Told His Cook
Arrange a ring of six large (not miniature) marshmallows, upright and well separated, Stonehenge style, on a microwave-safe dinner plate. Zap in the microwave oven on high, while watching through the window as the marshmallows balloon to several times their size. Stop the oven when they have developed brown, volcano-like holes on top, after about 1 1/2 minutes.
Remove the plate carefully (it will be quite hot) and place it on the counter to cool completely, during which time the marshmallows will deflate and flatten.
Remove each “zap” from the plate (it will be quite sticky), dip its bottom into a plate of confectioners’ sugar and place it on a serving dish or platter. It will be crisp as a meringue on the outside and chewy on the inside, with a layer of brown caramelized sugar in the middle as if it were an inside-out fire-roasted marshmallow.
How does it work? The microwaves’ energy converts water into steam, which fills the millions of air bubbles in the marshmallow foam and puffs them up, as if blowing up millions of little balloons. When the gelatin’s elasticity limit is exceeded, the steam breaks its way out though a hole it punches in the top.
Meanwhile, the dehydrated interior sugar caramelizes under the influence of the heat. Dehydration is the first step in the complex series of chemical reactions involved in caramelization, so the interior dehydrated sugar caramelizes first. The outer parts of the marshmallow, still saturated with steam, would not caramelize unless heated longer. As the marshmallows cool, the steam condenses and the foam collapses, trapping the caramelized layer in the middle.