In a small pot over medium-low heat, warm the peanut butter and maple syrup.
As the mixture gets warmer, stir the ingredients to combine. Remove from the heat and then add the vanilla and cinnamon, if using. Stir to combine.
In a large mixing bowl, pour the rice cereal and peanut butter mixture. Using a rubber spatula, stir to combine.
In a loaf pan or an 8 x 8-inch baking dish lined with parchment paper, pour the mixture and gently press to form it.
Let it sit at room temperature for 45 minutes, then refrigerate for 30 minutes or freeze for 15 to 20 minutes.
Cut into slices or squares and enjoy!