Mix the sugar and ground cinnamon in a small bowl.Rub together the flour, salt, 3 tbsp cinnamon sugar and butter with your fingers until crumbly.Pour the milk into a large saucepan and add the cinnamon stick or 1 tsp ground cinnamon. Bring to the boil over medium heat. Reduce the heat and simmer while slowly sprinkling the flour crumbs into the milk and lightly whisking.
Reduce the heat and simmer, while stirring or whisking continually, for about 10 minutes. Turn off the heat, allow to stand for 5 minutes and serve with the remaining cinnamon sugar. Enjoy!