Preheat your oven to 350°F (175°C).
Line your 9x13-inch baking dish with parchment paper.
In a large mixing bowl, whisk together the melted unsalted butter and peanut butter until smooth. Add the egg, light brown sugar, and vanilla extract. Mix until creamy.
In a separate bowl, whisk together the all-purpose flour and semi-sweet chocolate chips. Slowly add this mixture to the wet ingredients, stirring until just combined.
Gently fold in the unwrapped miniature peanut butter cups.
Pour the batter into the prepared baking dish and spread it out evenly. Sprinkle additional chocolate chips on top if desired. Bake for 25-30 minutes, or until the edges are golden.
Allow the bars to cool in the baking dish for about 15 minutes. Lift the bars out using the parchment overhang, place them on a wire rack to cool completely, then cut into squares.
