Heat 1 tablespoon of butter in a small skillet over medium heat. Add the diced apple, granulated sugar, and cinnamon. Cook for 3–4 minutes, stirring occasionally, until the apples are softened but not mushy. Set aside to cool.

Preheat your oven to 350°F and line an 8x8-inch baking pan with parchment paper, leaving some overhang for easy removal.

In a large mixing bowl, whisk together the melted butter and brown sugar until smooth. Add the egg and vanilla extract, mixing until fully combined.

In a separate bowl, whisk the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.

Spread half of the blondie batter evenly into the prepared pan. Spoon the cooled cinnamon apple filling over the batter in an even layer. Top with the remaining blondie batter, spreading gently to cover the apples.

Bake for 28–30 minutes, or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs. Let the blondies cool completely in the pan.

In a small bowl, whisk together the powdered sugar, maple syrup, and 1 tablespoon of milk. Add more milk, 1 teaspoon at a time, until the glaze is smooth and pourable.

Drizzle the maple glaze over the cooled blondies. Let the glaze set for about 15 minutes before cutting into squares.