Line an 8x8 baking pan with parchment paper. Set aside. In a medium bowl, stir together the crispy brown rice cereal and peanuts.
In a small saucepan, mix the peanut butter, maple syrup, and coconut oil until the mixture is smooth and the coconut oil is melted. Turn off the heat and add the vanilla, salt (if using), and cinnamon. Add the peanut butter mixture to the cereal mix, and stir until completely combined. There should be no dry cereal spots.
Spoon the mixture into the prepared pan, and press with the back of a spoon (or clean hands) until you have an even layer.
In a microwave (or double-boiler), melt the chocolate and coconut oil until just melted. Pour the chocolate over the peanut butter layer. Tilt the pan around until you have a smooth, even layer of chocolate. Sprinkle with rainbow sprinkles or flaky salt. Refrigerate for at least one hour.
Let the bars sit at room temperature for about 5-10 minutes. Cut into 16 even squares. Store in an airtight container for up to four days, or freeze for up to two months!