Prep the pan. Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving a little overhang for easy lifting. Lightly grease the parchment and sides.

Melt the butter. Melt butter in a small pot or microwave-safe bowl. Let it cool for 3–5 minutes so it doesn’t scramble the egg later.

Mix the wet ingredients. In a medium bowl, whisk the brown sugar and melted butter until glossy and combined. Add the egg and vanilla, and whisk until smooth and slightly thick.

Whisk the dry ingredients. In a separate bowl, combine flour, baking powder, baking soda, and salt. Stir to blend evenly.

Bring it together. Add the dry ingredients to the wet mixture. Stir with a spatula until just combined. The batter will be thick—avoid overmixing.

Fold in the chocolate. Stir in the chocolate chips (and nuts if using). Reserve a small handful of chips to sprinkle on top for a pretty finish.

Spread and level. Scrape the batter into the pan. Use the spatula to nudge it into the corners and smooth the top. Scatter the reserved chips over the surface.

Bake. Bake for 20–25 minutes, until the edges are set and golden and the center looks just set. A toothpick inserted in the center should come out with a few moist crumbs, not wet batter.

Optional finish. While still warm, sprinkle a pinch of flaky sea salt over the top. It sharpens the sweetness and adds crunch.

Cool and slice. Let the blondies cool in the pan on a rack for at least 20–30 minutes. Lift out with the parchment, then cut into 9 big squares or 16 smaller ones.