Lightly spray a full-size hotel pan with nonstick oil spray. Line it with a sheet of parchment paper. If your dehydrator is too small, use 2 half sheet pans.
Turn a dehydrator on to 50°C/120°F.
Combine both ingredients in a deep pot, much larger than what you need for the ingredients alone. It should be tall enough to contain the liquid when it comes to a rolling boil and should preferably not be too wide. Put the pot over medium-low to medium heat.
Using a handheld mixer, whip the mixture as it is warming up. It is important to get to the bottom of the pot and to make sure the mixture slowly rises in temperature. This procedure may take up to 30 minutes.
When the milk mixture just starts to come to a boil, take the pot off the heat. Spoon the foam onto the prepared hotel pan. There will be some unfoamed milk at the bottom of the pot; do not use it.
Put the hotel pan(s) in the dehydrator. Let the foam dry for at least 12 hours.
Remove the crispy foam from the dehydrator and the hotel pan and cut it into rectangles that measure 5cm/ 2in wide by 10cm/4in long. Try to cut it using a serrated knife. It may not cut into a very even shape because it is very brittle, which is not a problem; it can be semi-organically shaped.
Reserve the pieces in the dehydrator during service. After service, if there are any left over, transfer them to an airtight container with silica gel packs. They will keep indefinitely if stored under these conditions.