Heat oven to 400 degrees F.
Spray a 11 x 7 baking pan with non-stick spray generously. Set aside.
Combine the butter and brown sugar in a medium saucepan and cook on medium heat for about 3 to 5 minutes, or until the butter melts and the mixture is very well combined. Stir the mixture occasionally.
Increase heat a tiny little bit, and bring the mixture to a low boil, you want to cook it for about 2 to 3 minutes, just so it starts to thicken a bit and turn golden brown. Don't stir it during the process, but you can lightly swirl the pan. Keep a really close eye on it because it can go from a beautiful golden brown to a burnt mess in no time flat. As soon as it turns amber turn off the heat and swirl so it doesn't overcook.
Place the crackers in layers in the pan while the mixture is cooking. I used 30 crackers to line my 11 x 7 pan. It made two layers.
Pour the sugar mixture over top of the crackers as evenly as you can. Place the pan in the oven and cook for about 7 minutes. Remove from oven.
Toss peanut butter cups over top and cover the pan with foil to help them melt a bit more. Leave covered for about 5 to 7 minutes.
Uncover and top with walnut pieces.
Allow the bars to cool completely. Then break or cut into pieces.
Serve with a big old chocolate loving smile!