Preparation
Preheat your oven to 350°F. Line an 11x7 inch baking pan with aluminum foil or parchment paper, leaving overhang on the sides. If not using parchment, spray the pan well with cooking spray.
Making the Apple Filling
Peel apples and chop into small 1/4 inch pieces for even cooking.
In a medium saucepan over medium-low heat, melt 1 tablespoon butter. Add chopped apples, 2 tablespoons brown sugar, 1/2 teaspoon vanilla extract, and 1/2 teaspoon cinnamon. Stir to coat apples.
Cook for 3-4 minutes, stirring occasionally, until apples soften but still hold their shape.
Remove from heat and let cool completely before using.
Preparing the Blondie Batter
In a large mixing bowl, beat softened butter with an electric mixer on medium speed for about 1 minute until light and fluffy.
Add both dark brown sugar and granulated sugar. Mix on medium speed for 2 minutes until combined and creamy.
Add eggs one at a time, beating well after each addition until mixture is light and fluffy.
Add vanilla extract and salt. Mix until just combined.
In a separate bowl, whisk together flour, baking powder, and 1/2 teaspoon cinnamon until evenly mixed.
Add dry ingredients to wet ingredients. Stir with a wooden spoon or spatula until just combined. Batter will be very thick.
Assembling the Blondies
Take half the blondie batter and spread it into the prepared pan. Wet or spray your hands with cooking spray and press batter evenly across the bottom of the pan.
Spread cooled apple filling over the first batter layer, reaching all the way to the edges.
Drop spoonfuls of remaining batter over the apple layer. Use wet or greased hands to gently spread and press down to seal in the apples.
Baking
Bake on the center rack for 25-30 minutes until golden brown on top and edges start pulling away from sides.
Test with a cake tester or toothpick in the center - it should come out clean or with just a few moist crumbs. If wet batter remains, bake another 2-3 minutes.
Remove from oven and let cool in pan for 10 minutes.
Making the Maple Glaze
In a small saucepan over low heat, combine 2 tablespoons butter, maple syrup, 1/4 teaspoon vanilla extract, and 1/8 teaspoon cinnamon. Stir constantly until butter melts. Remove from heat immediately.
Sift powdered sugar into the warm maple mixture. Whisk vigorously until completely smooth.
Let glaze sit for 8-10 minutes off heat to thicken to drizzling consistency.
Final Assembly
Pour slightly thickened glaze over warm blondies. Use a spoon or offset spatula to spread evenly across the top.
Let cool completely for about 1 hour at room temperature until glaze sets.
Use foil or parchment overhang to lift blondies out of pan. Place on cutting board and slice into 12 squares with a sharp knife.