Warm most of the milk with the sugar in a pot without boiling.

Mash the rice flour and cornstarch with the remaining cold milk until lump-free, then add to the hot milk; cook over low heat, stirring constantly, until it thickens.

Stir in the vanilla, then divide the mixture among serving bowls.

Once the bowls reach room temperature, chill in the refrigerator for at least one hour; sprinkle with optional cinnamon before serving.