Whisk the corn starch with a little cold milk until smooth.

Stir the remaining milk and sugar together in a pan and heat until it starts to boil.

Whisk the corn starch mixture slowly into the boiling milk and cook over low heat, stirring constantly, for 12 more minutes until it thickens.

Stir the crushed mastic into the mixture and cook for one more minute until fully dissolved; keeping it from clumping takes some care.

Divide the mixture among bowls and, once at room temperature, chill in the fridge for at least 15 minutes, then top each with a spoonful of clotted cream before serving.