Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides for easy removal. Lightly grease any exposed parts of the pan.
Step 2: In a large mixing bowl, combine the melted butter and packed brown sugar. Whisk vigorously for 1-2 minutes until the mixture is smooth, glossy, and well combined.
Step 3: Add the egg, egg yolk, and vanilla extract to the butter-sugar mixture. Whisk until fully incorporated and the mixture is lightened slightly in color.
Step 4: In a separate small bowl, whisk together the all-purpose flour, baking powder, and salt until well combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Be careful not to overmix; a few streaks of flour are acceptable.
Step 6: Fold in your desired mix-ins, such as chocolate chips, chopped nuts, or white chocolate chunks, until evenly distributed throughout the batter.
Step 7: Pour the blondie batter into the prepared baking pan and spread it evenly with a spatula.
Step 8: Bake for 25-30 minutes, or until the edges are set and lightly golden brown, and the center is still slightly soft and looks just barely underbaked. A toothpick inserted into the center should come out with moist crumbs, not wet batter. Avoid overbaking for the chewiest result.
Step 9: Remove the pan from the oven and let the blondies cool completely in the pan on a wire rack. This allows them to set properly and develop their signature chewy texture.
Step 10: Once completely cool, use the parchment paper overhangs to lift the blondies out of the pan. Cut into 16 squares and serve. Store leftovers in an airtight container at room temperature for up to 3-4 days.