Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large mixing bowl, whisk together the melted butter, light brown sugar, and pure maple syrup until well combined and smooth.
Step 3: Add the egg, vanilla extract, and maple extract. Whisk vigorously for 1-2 minutes until the mixture lightens in color and becomes slightly fluffy.
Step 4: Gently stir in the all-purpose flour, baking powder, and sea salt using a spatula. Mix just until no streaks of flour remain—do not overmix.
Step 5: Fold in the toasted chopped walnuts. Spread the thick batter evenly into the prepared baking pan.
Step 6: Bake for 22-26 minutes, or until the edges are golden and a toothpick inserted into the center comes out with just a few moist crumbs. Do not overbake! Let the maple walnut blondie bars cool completely in the pan before slicing.