Preheat oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch square baking pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly grease the parchment and any exposed sides of the pan.

Melt the butter in a microwave-safe bowl or in a small saucepan on the stovetop. Let it cool for about 5 minutes until warm but not hot, so it doesn’t scramble the eggs.

In a large mixing bowl, whisk together the melted butter, peanut butter, brown sugar, and granulated sugar until smooth and glossy, 30–45 seconds.

Whisk in the eggs one at a time, then whisk in the vanilla extract until the mixture is smooth and thick. Do not overmix.

In a separate small bowl, whisk together the flour, baking powder, and salt until well combined and no baking powder lumps remain.

Add the dry ingredient mixture to the peanut butter mixture. Using a spatula or wooden spoon, gently fold just until no streaks of flour remain. The batter will be thick, similar to soft cookie dough.

Fold in the chocolate chips or chunks, if using, reserving a small handful to sprinkle over the top before baking.

Scrape the batter into the prepared pan. Use an offset spatula or the back of a spoon to press it into an even layer, pushing it into the corners. If the batter is sticky, lightly grease your spatula or fingers.

Sprinkle the reserved chocolate chips and chopped peanuts or flaky sea salt (if using) over the top. Bake for 22–28 minutes for an 8x8-inch pan, or 18–23 minutes for a 9x9-inch pan, until the edges are set and lightly golden and the center looks just barely underdone. A toothpick inserted near the center should come out with a few moist crumbs but not wet batter.

Place the pan on a wire rack and let the blondies cool in the pan for at least 45–60 minutes so they can set and slice cleanly.

Use the parchment overhang to lift the blondies out of the pan. Slice into 12 large or 16 smaller bars. Serve at room temperature, or slightly warm with vanilla ice cream and a drizzle of melted peanut butter if desired.