Preheat the oven to 300 degrees F. Slice the plums (peels on) and coat in the brown sugar. Roast for about 15 - 20 minutes, or until softened and juices are seeping out.
While the plums are roasting, mix the gelatin with two tablespoons of water and set aside.
Mix together the milk, cream, sugar, and salt together.
Gently heat and mix until the sugar is dissolved. Do not let the mixture boil.
Add in the plums and transfer to a blender. Blend well until the plums are completely blended in without any chunks of plum.
Mix in the gelatin until it is completely incorporated.
Chill the mixture until it is very cold, at least 40 degrees F.