Stir brown sugar and corn syrup together in a medium saucepan.
Bring to a boil over medium heat, stirring regularly.
Once at a boil, cook for 1 minute, stirring the entire time.
Remove from heat.
Stir in the peanut butter, vanilla, and salt.
Pour the peanut butter mixture over the cereal mixture.
Carefully toss to coat (a silicone spatula works well for this).
Pour the mixture into the prepared pan, gently pressing the mixture into the pan.
Let cool to room temperature.
Once the treats are at room temperature, drizzle with melted chocolate.
Let the chocolate set up before cutting and serving.
Notes
Rice Chex: Feel free to use a similar cereal, like Corn Chex, instead.
Peanuts: The salt on the peanuts helps to add some balance to the bars. Salted or lightly salted peanuts will work. You can also use unsalted peanuts and then adjust the amount of salt to taste.
Brown sugar: Light brown sugar or dark brown sugar will work. Dark brown sugar will give your bars slightly richer flavor.
Corn syrup: I haven't tried using any substitutes for the corn syrup.
Peanut butter: I use regular creamy peanut butter, not natural peanut butter.
Salt: Feel free to add more or less salt depending on how sweet and salty you want the bars to be.
Baking chocolate: If you want the bars to be sweeter, use milk chocolate. If you'd like bars that aren't as sweet, use dark or bittersweet chocolate.