Brown the Butter: Melt 1 cup unsalted butter in a wide stainless steel saucepan over medium heat. Swirl occasionally for 5 to 7 minutes as the butter melts, foams, and crackles. When the crackling subsides and amber milk solids settle at the bottom with a rich hazelnut aroma, immediately spoon 2 tablespoons of hot brown butter into a small skillet for the apples. Pour the remaining brown butter into a heatproof bowl to cool for 10 minutes.

Sauté Spiced Apples: Place the skillet with the 2 tablespoons reserved brown butter over medium heat. Add the diced apples, 1 teaspoon cinnamon, nutmeg, and 1 tablespoon maple syrup. Sauté for 4 to 5 minutes until tender-crisp and juices reduce. Remove from heat and allow to cool completely (cooking off excess water is the secret to chewy, fudgy blondies rather than cakey or soggy bars).

Preheat & Prepare Pan: Preheat your oven to 350°F (175°C). Line an 8x8-inch metal baking pan with unbleached parchment paper, leaving a 2-inch overhang on two sides for effortless lifting after baking.

Whisk Sugars & Eggs: In a large ceramic mixing bowl, whisk together the cooled brown butter, dark brown sugar, and granulated sugar for 1 minute until well combined and glossy. Add the room-temperature eggs, egg yolk, and alcohol-free vanilla bean paste. Whisk vigorously for a full 2 minutes until the mixture turns pale, thick, and ribbon-like (this aerates the sugar-egg emulsion and guarantees the signature shiny, crackly crust).

Fold Dry Ingredients & Apples: Using a flexible silicone spatula, gently fold in the all-purpose flour, fine sea salt, baking powder, and remaining 1/2 teaspoon cinnamon just until barely combined. Gently fold in the cooled sautéed apples along with any caramelized skillet juices until evenly distributed. Do not overmix.

Bake to Golden Perfection: Spread the thick batter evenly into the prepared pan and smooth the top. Sprinkle evenly with the cinnamon sugar mixture. Bake on the center oven rack at 350°F for 28 to 30 minutes, until the edges are golden and set, the top is crackly, and a toothpick inserted into the center comes out with moist, fudgy crumbs (never wet batter).

Cool, Slice & Finish: Remove from the oven and immediately scatter flaky Maldon sea salt across the top. Let the blondies cool completely in the pan on a wire cooling rack for at least 1 hour (cooling is essential to let the brown butter crumb solidify into clean, chewy squares). Use the parchment overhang to lift the slab onto a cutting board and slice into 16 neat squares.