• Preheat oven to 350°F (177°C). Adjust the oven rack to the center position. Line a 9x13-inch metal or glass baking pan with parchment paper, leaving an overhang on the sides for easy removal.
• In a large bowl, whisk together the melted butter and brown sugar until combined.
• Whisk in the peanut butter until smooth. Add the eggs, egg yolk, and vanilla extract, whisking until fully incorporated.
• Add the flour, baking powder, and salt. Fold with a rubber spatula until just combined. The batter will be very thick.
• Fold in 1 ½ cups of the peanut butter chips, reserving the remaining ¼ cup for the topping.

Peanut Butter Swirl

• In a small microwave-safe bowl, warm the peanut butter for 10 to 15 seconds, just until loosened and easy to drizzle. Do not overheat.

Assemble & Bake

• Spread the batter evenly into the prepared pan.
• Dollop the warmed peanut butter over the top in small spoonfuls. Use a toothpick or butter knife to gently swirl it into the batter, creating a marbled pattern. Be careful not to over-swirl.
• Sprinkle the remaining ¼ cup peanut butter chips over the top.
• Bake for 30 to 35 minutes, checking at the 30-minute mark. A toothpick inserted into the center should come out with a few moist crumbs but no wet batter. The edges will be set and lightly golden, while the center may still look slightly underbaked.
• Let the blondies cool completely in the pan on a wire rack. Using the parchment overhang, lift them from the pan and cut into squares before serving.