Preheat the oven to 350°F and prepare an 8x8-inch baking dish by lining it with parchment paper (or lightly grease and flour). (You could use a 9x13-inch baking dish for more servings, but the bars will be thinner.)
Using a whisk, combine the melted butter and brown sugar in a large bowl. Mix well.
Add the eggs and the egg yolk, one at a time, stirring well after each addition, scraping the sides of the bowl as needed.
Whisk in the vanilla extract.
In a separate, medium-sized bowl, whisk together the flour, cornstarch, baking powder, and salt.
Using a rubber spatula, gradually stir the flour mixture into the butter and egg mixture until completely combined.
Fold in diced apples.
Spread the batter evenly into the prepared pan.
Bake for 50-55 minutes, or until the edges just begin to turn golden brown and there is no jiggle in the bars. (If you are using a 9x13-inch pan, bake for about 45 minutes.)
Allow blondies to cool before topping with maple glaze.