Preheat the oven to 350 degrees. Line an 8 x 8 baking dish with parchment paper and spray with cooking spray.
Melt the butter and peanut butter together in a microwave safe bowl. Whisk together to cool, and add the brown sugar, egg, and vanilla extract. Stir to combine.
Add the flour, cookies, and other mix in ingredients except for the M&Ms. Use a spatula or a mixer to gently combine. Pour the mixture into the baking dish and press evenly so that you have one layer. Add the M&Ms over top, gently pressing them into the batter with your fingers.
Bake the bars for 20 - 22 minutes, or until the batter is golden brown. Remove from the oven and let cool for 5 minutes before removing the parchment paper. Cool the bars another 20 minutes before slicing and serving. I recommend slicing into 2 x 2 inch pieces, as these bars are very decadent.
*The filling choices are just the ones I used, you can use 2 ½ cups of any type of cookie or candy you have around.
Recipe by Cooking in Red Socks at http://www.cookinginredsocks.com/2014/03/31/kitchen-sink-cookie-bars/