Line the interior of a glass baking dish with Butter (1/2 tablespoon) and sprinkle with spoonful of Powdered Confectioners Sugar (to taste).
Use a mortar and pestle to grind the Mastic (1 piece) with Granulated Sugar (1 teaspoon) until it is broken down into fine pieces.
In a bowl, combine Corn Starch (1/2 cup) and Rice Flour (1/2 cup). Add Whole Milk (1 cup) to dissolve and whisk to form a thick mixture.
In a large pot over the stove, place the rest of the Whole Milk (3 1/4 cup), Granulated Sugar (1 cup), starch and flour mixture, Vanilla Extract (1/4 teaspoon) and crushed mastic. Cook over medium-low heat, whisking constantly, for about 10 minutes until it thickens to a pudding consistency.
Now pour the pudding into the prepared glass tray, and set over a low flame. Using oven mitts, move the tray around every minute or so when you see that a part of the bottom of the pudding has become a dark brown.
Continue to move the tray around the flame so that the bottom is relatively evenly caramelized.
Place in cold water bath or directly into the refrigerator. Eat once completely cooled in about 4 to 5 hours, which will make removing it from the pan much easier. Serve and enjoy!