Prepare a 9-inch square pan by greasing it well with butter.
In a medium saucepan, add the brown sugar and corn syrup, and stir to combine.
Place the pan over medium heat. Bring the mixture to a boil, stirring very often, and let it boil for 1 minute.
Remove the pan from the heat, and stir in the peanut butter. When the mixture is fully combined, pour it over the cereal. Stir to combine very well, making sure no dry pockets of cereal remain.
Pour the mixture into the prepared pan, and use a spatula the spread out the mixture. Don't pack it down -- just spread it out.
Let the cereal bars cool for 30 minutes before serving.
Store leftovers covered at room temperature for up to 5 days.
Notes
You can sub the corn syrup for 1/3 cup honey, if you prefer.
Be sure not to "smoosh" the treats into the pan so they stay light and fluffy.
Peanut butter chocolate candies mixed in make it an extra fun treat.
Store them in an airtight container at room temperature.