Preheat the Oven:
Preheat your oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal.

Prepare the Blondie Base:
In a large mixing bowl, whisk together the melted butter and brown sugar until smooth and well combined. Add the eggs, one at a time, and beat until fully incorporated. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Make the Peanut Butter and Chocolate Swirls:
In a microwave-safe bowl, heat the peanut butter for 20-30 seconds to soften it slightly, then stir until smooth.
In another microwave-safe bowl, melt the chocolate chips in 20-second intervals, stirring in between, until smooth and melted.

Assemble the Blondies:
Pour the blondie batter into the prepared baking pan and spread it evenly.
Drop spoonfuls of peanut butter and melted chocolate on top of the batter. Use a butter knife or skewer to swirl the peanut butter and chocolate into the batter in a marble pattern.

Bake the Blondies:
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. The edges should be golden and the center should be set.

Cool and Serve:
Allow the blondies to cool completely in the pan before lifting them out using the parchment paper. Slice into squares and enjoy!