Preheat the oven to 350° F. Line an 8" x 8" pan with aluminum foil and spray with cooking spray. Set aside.
Bake for 22-25 minutes, or until the edges of the bars are golden brown. Wait about 5 minutes, and then quickly top the bars with the remaining 15 Kit Kats (working quickly will prevent them from melting too much). Press the Kit Kats onto the tops of the bars so they begin to stick and melt, and then transfer the pan to the fridge to cool for about 30 minutes before serving. Enjoy!