1. In a medium saucepan, mix sugar and rice flour with a whisk until combined. Slowly add milk, while whisking, until mixture is smooth.
2. Over medium heat, bring mixture to almost a boil, whisking constantly. The mixture will slowly get thicker as it heats.
3. Once you see a few small boils, set a timer for 2 minutes. It is important to continue whisking constantly.
4. When 2 minutes is up, turn the heat off. The mixture should be thick enough to coat the back of a spoon. Divide among 8 small cups or ramekins (or fewer, if you want larger portions). Sprinkle with cinnamon, if desired.
5. Allow to cool completely in the refrigerator, preferably overnight, before eating. Sutlach will keep about 3 days, if it lasts that long!