In a large saucepan whisk together the milk and the cornstarch thoroughly, for about one minute until no lumps remain.
Place on medium low heat, add the sugar and crushed mastic and whisk to combine. Add cinnamon stick if using.
Continue to cook and whisk until thickened into a consistency that's thicker than hot chocolate but looser than pudding, or to the consistency of your taste! This can take anywhere from 6-10 minutes. Don't be tempted to increase the heat because the milk can easily burn.
Once thick and creamy, pour sahleb into serving cups of choice, and top with garnishes of your choice. Traditionally, nuts are common (pistachio or walnut), as well as cinnamon powder, and sometimes raisins and shredded coconut. Enjoy warm, so good!