For the kaffir lime salt: Grind together the dehydrated limes and the salt.
For the rendang: Place the sweetbreads in a saucepan and cover with water. Bring to a boil, then lower the heat to medium and simmer for 30 minutes to blanch. Shock in ice water, and then peel the membrane off. Run the sweetbreads under cold water until the water runs clear.
In a saute pan over medium heat, add the oil. Then add the onions, lemongrass and sweat until aromatic, 3 to 5 minutes. Then add the belacan, arbol chiles, coriander seeds, cloves and cinnamon stick. Cook until the aroma from the belacan is cooked out, about 5 minutes. Deglaze with the coconut milk. Add the salt and simmer for about 1 hour on low heat, uncovered. Add the blanched sweetbreads and cook for another 30 minutes.
For the rice espuma: Heat a saucepan over medium heat and add the rice. Toast until you get a toasted flavor, 20 to 30 seconds. Then add in the milk, palm sugar and cinnamon stick. Bring to a simmer, and then remove from the heat. Blend the mixture until very fine (see Cook's Note, below). And then strain through a fine mesh sieve.
When ready to serve, froth with a foam gun, after discharging twice, and spoon over the redang. Garnish with kaffir lime salt.
Cook's Note: When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.