In a medium mixing bowl, whisk together the dry ingredients: flour, cocoa powder, baking soda, and salt. Set aside.
In a large mixing bowl, place the unsalted butter and both granulated sugar and dark brown sugar. Cream until light and fluffy.
Add the egg and vanilla extract to the mixture and mix just until combined.
With the mixer on low speed, add the dry ingredients and mix just until combined.
Fold in the old-fashioned oats, Cheerios, and chocolate chips.
Cover the dough and refrigerate for 2 to 3 hours.
Preheat your oven to 350 degrees Fahrenheit.
Line a 13" x 9" baking pan with parchment paper.
Scrape the dough into the prepared pan and smooth it out with an offset spatula.
Bake for 16 to 19 minutes, until the edges appear set and dry.
Let cool completely in the pan.
Drizzle warm peanut butter over the cooled bars and let sit for 15 minutes until set. Cut into bars.
