In a large bowl, combine the cooked rice, softened butter, honey, vanilla extract, salt, and cinnamon (if using). Mix well until the ingredients are fully incorporated and the rice is coated evenly.
Line a baking dish (8x8 inches) with parchment paper for easy removal. Transfer the rice mixture into the dish, pressing it down firmly to create an even layer.
Cover the dish with plastic wrap and refrigerate for at least 1 hour to allow the mixture to firm up.
Once chilled, remove the rice cake from the dish using the parchment paper. Cut into squares or rectangles.
If desired, sprinkle chopped nuts or seeds on top for added texture and flavor. Serve immediately or store in an airtight container in the refrigerator for up to 3 days.