Add milk, sugar, vanilla, cinnamon, and lemon zest to a large, pot. Bring to a boil whisking regularly then take the pot off the heat (be careful, when the milk comes to a boil it will immediately boil over so take it off the heat immediately, also make sure your pot is large enough).
Add semolina, whisking constantly at the same time (to avoid lumps).
Place the pot back on the burner, bring to a boil over medium heat, then cook for 1 minute over low heat (whisking).
Take the pot off the heat and cover with a lid or a plate. Let stand for 5 minutes.
Stir in the heavy cream and butter.