Preheat your oven to 350°F (175°C). Grease your 9x9-inch baking pan or line it with parchment paper for easier removal later.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until well combined. Set aside.
In a large mixing bowl, combine the melted butter and brown sugar. Stir until the mixture is smooth. Then add the egg and vanilla extract, mixing until fully incorporated.
Gradually add the dry ingredient mixture to the wet ingredients, stirring until just combined. Be careful not to overmix!
Gently fold in the mini peanut butter cups and semisweet chocolate chips using a spatula. This is where the magic happens!
Pour the batter into the prepared baking pan, spreading it evenly. Drizzle the creamy peanut butter over the top and use a knife to swirl it through the batter. Bake for 20-25 minutes, or until the edges are golden and a toothpick inserted into the center comes out mostly clean.
Allow the bars to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely. Cut into squares and serve with a glass of milk or your favorite beverage!