Step 1: Preheat your oven to 350°F (175°C). Grease an 8×8 inch baking pan and line it with parchment paper, leaving an overhang on two sides for easy removal.
Step 2: In a large mixing bowl, combine the melted butter and dark brown sugar. Whisk vigorously for 1-2 minutes until the mixture is smooth and well-combined.
Step 3: Add the egg and vanilla extract to the butter mixture. Whisk until the mixture looks glossy and the egg is fully incorporated.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, ground cinnamon, baking powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients. Use a rubber spatula to fold the mixture together until just a few streaks of flour remain.
Step 6: Gently fold in the diced apples until they are evenly distributed throughout the thick batter.
Step 7: Transfer the batter to the prepared baking pan. Use a spatula to spread it into an even layer, reaching all the way to the corners.
Step 8: Bake in the center of the preheated oven for 30-35 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with just a few moist crumbs.
Step 9: Remove the pan from the oven and place it on a wire rack. Allow the blondies to cool completely in the pan before lifting them out using the parchment overhang.
Step 10: Slice into 12 bars and serve. For an extra touch, dust with powdered sugar or a drizzle of maple glaze.