Step 1: Preheat your oven to 350°F (175°C) and grease a 9x9 inch square baking pan, or line it with parchment paper for easy removal.
Step 2: In a large mixing bowl, whisk together the melted butter and brown sugar until smooth and well combined.
Step 3: Add the egg and vanilla extract to the butter mixture, whisking vigorously for about one minute until the batter looks slightly glossy.
Step 4: In a separate bowl, whisk together the flour, 2 teaspoons of cinnamon, baking powder, and salt.
Step 5: Gradually fold the dry ingredients into the wet ingredients using a spatula until just combined. Do not overmix.
Step 6: Gently fold in the diced apples until they are evenly distributed throughout the thick batter.
Step 7: Spread the batter into the prepared baking pan, smoothing the top with your spatula.
Step 8: Sprinkle the cinnamon-sugar topping evenly over the surface of the batter.
Step 9: Bake 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 10: Allow the blondies to cool completely in the pan before slicing into 16 squares.