Crush the mastic with a bit of sugar in a mortar and pestle until finely ground.
Combine the starch and the flour in a pot. Add about a glass of the milk and mix well until dissolved. Make sure no lumps remain.
Add the remaining milk, ground mastic and the sugar to the pot, again mix well.
Start cooking over small-medium heat, while continuously mixing. When you start obtaining a pudding like texture and bubbles start coming up, close the heat. Add the vanilla.
If possible, pour the mixture into a mixer and constantly mix the mixture at lowest possible speed, until you need to use the pudding. If not set aside with lid covered and mix once in a while.
Grease your pan/Pyrex dish with butter.
Sprinkle with powder sugar all around evenly. Add on top about a cup of the pudding mixture. Start caramelizing over medium heat on the stove. Moving your pan/dish around will help this happen evenly . The bottom will soon caramelize and turn brownish, be patient. When you decide it is browned enough remove from the stove.
Slowly pour the remaining pudding on top do not damage the burned bottom. Let cool for about five minutes then prepare another tray with water and place the pan/dish into it so that serving will be easier. Keep the pudding tray in the water tray for about 10 minutes then fridge it until serving.
To serve, use a knife to cut the pudding to rectangulars. Use a spatula to take each piece out. You can either serve up-side down or make it round again with the burnt side up.
Sprinkle some cinnamon. Enjoy.