Step 1: Preheat your oven to 350°F (175°C) and line an 8x8 inch square baking pan with parchment paper, leaving a slight overhang on the sides for easy removal.
Step 2: In a medium saucepan, melt the butter over medium heat. Continue cooking, swirling occasionally, until the butter browns and develops a nutty aroma. Remove from heat and let cool for 5 minutes.
Step 3: In a large bowl, combine the browned butter and brown sugar. Whisk until smooth and well combined.
Step 4: Add the egg and vanilla extract to the sugar mixture. Whisk vigorously for about 1-2 minutes until the batter looks light and shiny.
Step 5: Gently fold in the flour, cinnamon, and salt using a spatula. Do not overmix; stop once no streaks of flour remain.
Step 6: Carefully fold the diced apples into the batter until evenly distributed.
Step 7: Spread the batter into the prepared baking pan, smoothing the top with a spatula. Bake for 30-35 minutes, or until the edges are golden and a toothpick inserted into the center comes out with just a few moist crumbs.
Step 8: Allow the blondies to cool completely in the pan before lifting them out and slicing into squares.