Preheat your oven to 350°F (175°C). This ensures that your bars bake evenly and perfectly.
Grease a 9x13 inch baking pan with a bit of butter or line it with parchment paper for easy removal later.
In a large mixing bowl, combine the melted butter, creamy peanut butter, granulated sugar, brown sugar, vanilla extract, and the egg. Use a spatula or whisk to mix until smooth and creamy.
In another bowl, whisk together the all-purpose flour, baking soda, corn starch, and salt. Make sure there are no lumps.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the semi-sweet chocolate chips and the chopped Reese's peanut butter cups. This is where the magic happens, and your bars start to come to life!
Pour the batter into the prepared baking pan, spreading it evenly with your spatula.
Place the pan in the preheated oven and bake for about 25-30 minutes, or until the edges are golden and a toothpick inserted in the center comes out clean. You want the center to be slightly underbaked for that chewy texture.
Once baked, remove the pan from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely. Once cooled, cut into squares or rectangles, and serve!
