Add the peanut butter and maple syrup to a saucepan and heat over medium heat on the stove. Stir well until the maple syrup has melted into the peanut butter. The mixture will be thick but smooth. Remove from the heat and set aside.
In a large bowl, toss together the Cheerios and popcorn. Pour half of the peanut butter mixture over top and mix to distribute with a spoon or spatula. Pour the other half of the peanut butter mixture into the bowl and mix again until all of the Cheerios and popcorn are coated.
Line an 8 X 8 inch baking pan with parchment paper and press the Cheerio mixture into the pan tightly. Make sure to press it into the sides and corners well. Set aside.
In a sauce pan, melt the dark chocolate chips. If needed, add a tablespoon of coconut oil to thin it out. Pour the chocolate over the peanut butter Cheerio bars. Tap the pan on the counter a few times to even out the chocolate. Place the pan in the fridge until the chocolate is just set (about 20 minutes). Try not to leave the pan in the fridge for too long or the chocolate will become too hard to slice. You want the chocolate to be slightly soft.
Once the chocolate has set, remove the peanut butter Cheerio bars from the pan and flip the entire thing upside down on a cutting board so the chocolate is on the bottom. Use a thin sharp knife to slice 8 bars out of the square and enjoy!