Prepare Your Pan and Oven:

1. Position your oven rack in the center of the oven and preheat to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper, leaving about 2 inches of overhang on two opposite sides to act as handles for easy removal later. Lightly grease the parchment paper with butter or non-stick spray—this extra insurance prevents any sticking.

Melt the Butter: 2. Cut the butter into tablespoon-sized pieces and place in a medium saucepan over medium-low heat. Stir occasionally as the butter melts completely, then immediately remove from heat. Alternatively, melt in a microwave-safe bowl in 20-second intervals, stirring between each interval until completely melted. Let the melted butter cool for about 5 minutes—it should be warm but not hot enough to scramble the eggs.

Mix Wet Ingredients: 3. Transfer the melted butter to a large mixing bowl (or use the same bowl if you melted butter in the microwave). Add both the brown sugar and white sugar to the warm butter. Using a wooden spoon or whisk, stir vigorously for about 1 minute until the mixture is smooth, glossy, and well combined. The sugars won’t dissolve completely, but they should be thoroughly mixed with no dry spots or butter streaks visible.

Add Eggs and Vanilla: 4. Add the whole egg, egg yolk, and vanilla extract to the butter-sugar mixture. Whisk or stir vigorously for about 30-45 seconds until the mixture becomes lighter in color, slightly thickened, and completely smooth. The mixture should look glossy and emulsified. This step is crucial for creating that signature dense, chewy texture, so don’t rush it.

Combine Dry Ingredients: 5. In a separate small bowl, whisk together the flour, baking powder, and salt. This ensures the leavening and salt are evenly distributed throughout the flour, preventing pockets of salty or flat-tasting blondies.

Mix Batter: 6. Pour the flour mixture into the wet ingredients all at once. Using a rubber spatula or wooden spoon, gently fold and stir the mixture together. Mix just until no flour streaks remain and you have a thick, smooth, uniform batter—this should take about 20-30 seconds of gentle stirring. The batter will be quite thick, similar to cookie dough consistency. Be careful not to overmix, which can develop gluten and create tough, cakey blondies instead of chewy ones.

Add Mix-ins: 7. Reserve about 2 tablespoons of chocolate chips for topping, then fold the remaining chocolate chips (and nuts if using) into the batter. Use a gentle folding motion to distribute them evenly throughout without overmixing the batter.

Spread in Pan: 8. Scrape the thick batter into your prepared pan. Use your spatula to spread it evenly into all corners—the batter is quite thick, so you’ll need to work it into an even layer. Don’t worry about it looking perfect; it will spread and level during baking. Smooth the top as best you can, then sprinkle the reserved chocolate chips over the surface for a pretty presentation.

Bake to Perfection: 9. Place the pan in your preheated oven and bake for 25-30 minutes. The blondies are done when the edges are set and lightly golden brown, the center looks just barely set (it may jiggle very slightly when you gently shake the pan), and a toothpick inserted into the center comes out with a few moist crumbs attached—not wet batter, but definitely not clean. Start checking at 25 minutes, as overbaking is the enemy of chewy blondies.

Cool Completely: 10. Remove the pan from the oven and place it on a cooling rack. Let the blondies cool completely in the pan—this is absolutely essential and takes about 1-2 hours. Blondies are very soft and fragile when warm and need time to set up properly. For the cleanest cuts and best texture, many bakers actually refrigerate the cooled blondies for 30 minutes before slicing.

Cut and Serve: 11. Once completely cool, use the parchment paper overhang to lift the entire blondie slab out of the pan onto a cutting board. Use a sharp knife (or even a plastic knife, which cuts beautifully without creating crumbs) to cut into 16 squares (4×4 grid). For the cleanest cuts, wipe your knife clean with a damp paper towel between each cut. Alternatively, run your knife under hot water, dry it, then slice—the warm blade cuts through more cleanly.