In a saucepan over medium-high heat, mix the rice, milk, and salt. Cook until it starts to thicken (for around 15-20 minutes) while stirring often to prevent the rice from burning. Add the sugar and stir until melted.
Take about ¼ cup of the hot rice mixture and rapidly mix it into the beaten eggs. Once tempered, add to the hot pudding and rapidly whisk them in. Cook for a few more minutes, stirring constantly until it thickens.
Remove the pudding from the heat and stir in the butter and vanilla extract. Season with cinnamon to taste.