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

In a microwave-safe bowl or small saucepan, melt the butter until just melted but not sizzling. Let cool for 3–5 minutes so it does not cook the eggs later.

In a medium mixing bowl, whisk together the melted butter and peanut butter until smooth and glossy. Add the brown sugar and granulated sugar and whisk until well combined and slightly thickened.

Whisk in the egg, egg yolk, and vanilla extract until the mixture is smooth and slightly lighter in color.

In a small bowl, whisk together the flour, baking powder, and fine sea salt until evenly combined.

Switch to a spatula and fold the dry ingredients into the wet mixture, stirring just until no dry streaks of flour remain. The batter will be thick, similar to cookie dough.

Fold in the chocolate chips or peanut butter chips, reserving a small handful to sprinkle on top if desired.

Scrape the thick batter into the prepared 8x8-inch pan. Use a spatula or the back of a spoon to press it into an even layer, pushing it into the corners so it bakes level. Sprinkle reserved chips on top if using.

Bake for 22–26 minutes, until the edges are set and lightly golden and the center looks matte but still slightly soft. A toothpick inserted in the center should come out with a few moist crumbs, not wet batter.

Place the pan on a wire rack and cool the blondies completely, about 1–1.5 hours. If using, sprinkle flaky sea salt on top right after baking while they are still warm. For cleaner slices, chill in the fridge for 30 minutes, then use the parchment to lift the slab out and cut into 16 small squares or 9 large bars.