Melt the butter in the cooker and stir in the rice, coating it with the butter.
Pour in the fresh milk and water, and then stir in the sugar and salt. Close the lid and bring to pressure. Cook for 8 minutes on High or 15 minutes on Low. Let the pressure lower naturally, and then remove the lid.
In a small bowl mix together the egg, evaporated milk and vanilla.
Stir in a little of the hot liquid from the cooker, then add the mixture to the cooker. Cook, uncovered, over medium heat, stirring constantly with a wooden spoon until the rice pudding just barely begins to bubble.
Remove immediately from the flame. Cool 10 minutes, stirring occasionally.
Transfer to individual dessert bowls, continue cooling, then refrigerate. Serve sprinkled with cinnamon.