Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan or line it with parchment paper for effortless removal of the blondies after baking.
In a large mixing bowl, combine the melted unsalted butter and creamy peanut butter. Use a whisk or spatula to mix until the two ingredients are smooth and well blended.
Add the packed brown sugar into the peanut butter mixture, stirring vigorously until fully incorporated and smooth.
One at a time, beat in the large eggs, mixing well after each addition. Then, add the vanilla extract and ensure the batter is smooth and creamy before moving on.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt until evenly mixed.
Gradually fold the dry flour mixture into the peanut butter batter using a rubber spatula or wooden spoon. Mix just until combined; be careful not to overmix as this can affect texture.
Gently fold in the coarsely chopped chocolate peanut butter cups and optional chocolate chips, ensuring they are evenly distributed throughout the batter.
Pour the thick batter into the prepared baking pan, using a spatula to spread it evenly across the pan.
Place the pan in the preheated oven and bake for 20-25 minutes. Check for doneness; the edges should be golden brown, and a toothpick inserted into the center should come out with a few moist crumbs (not wet batter).
Once baked, remove from the oven and allow the blondies to cool in the pan for about 10 minutes. After that, carefully transfer them to a wire rack to cool completely.
Notes
For an extra touch of indulgence, drizzle with melted chocolate or serve alongside a scoop of creamy vanilla ice cream.