Line an 8x8-inch baking pan with parchment paper, letting the ends overhang for easy removal. Lightly spray the parchment with nonstick cooking spray. Preheat your oven to 350°F.

In a large microwave-safe bowl, melt the 1/2 cup butter in the microwave or in a small saucepan over medium heat. Continue cooking until the butter turns a nutty golden brown, swirling occasionally. Remove from heat and let cool for 2–3 minutes.

Stir the brown sugar into the browned butter until well combined. Add the egg, vanilla, salt, and cinnamon, and mix until smooth. Fold in the flour just until no streaks remain. Spread the batter evenly into the prepared pan.

Bake the blondie base for 15 minutes, while you prepare the apple filling and streusel.

In the same bowl used for the blondies (no need to wash), combine the diced apples, brown sugar, flour, and cinnamon. Microwave on high for 5 minutes, stirring halfway through, until the apples are tender and the mixture is syrupy. Set aside.

In a separate bowl, combine the cold butter cubes, brown sugar, oats, flour, and cinnamon. Use your fingers, a fork, or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces.

After the blondie base has baked for 15 minutes, remove it from the oven. Carefully spread the apple filling evenly over the warm base. Sprinkle the streusel topping over the apples, gently pressing it down with your fingers. Return to the oven and bake for another 18–20 minutes, until the streusel is golden brown and the edges are bubbling.

Place the pan on a wire rack and let the bars cool completely to room temperature. Then refrigerate for at least 2–3 hours, or until fully set. Use the parchment slings to lift the bars out of the pan, then slice into 12 squares.