Toasted Meringue Hazelnut Blondies

Ingredients

• Nonstick vegetable oil spray (for pan)
• 2 cups blanched hazelnuts
• 1 cup (2 sticks) unsalted butter
• 2 large eggs, room temperature
• 1 large egg yolk, room temperature
• 1¼ cups (250 g) granulated sugar
• ¼ cup (packed; 50 g) light brown sugar
• 2 tsp. vanilla bean paste or vanilla extract
• 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt
• ¾ tsp. instant espresso powder (optional)
• 1½ cups (187 g) all-purpose flour
• 3 large egg whites
• 1 cup (200 g) granulated sugar
• 1 tsp. vanilla bean paste or vanilla extract
• ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, plus more
• Flaky sea salt
• A kitchen torch

Steps

1. Place a rack in middle of oven; preheat to 325°. Lightly coat a 9×9" pan with nonstick vegetable oil spray. Line pan with parchment paper, leaving overhang on two sides. Toast 2 cups blanched hazelnuts on a rimmed baking sheet, tossing halfway through, until golden brown and fragrant, 15–18 minutes. Let sit until cool enough to handle, then coarsely chop; set aside.
2. Cook 1 cup (2 sticks) unsalted butter in a small saucepan over medium heat, stirring often, until butter foams, then browns (do not burn), 7–9 minutes. Remove from heat.
3. Whisk 2 large eggs, room temperature, 1 large egg yolk, room temperature, 1¼ cups (250 g) granulated sugar, ¼ cup (packed; 50 g) light brown sugar, 2 tsp. vanilla bean paste or vanilla extract, 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, and ¾ tsp. instant espresso powder (if using) in the bowl of a stand mixer fitted with the whisk attachment on medium-high speed until mixture is pale and doubled in volume, 6–9 minutes (mixture will form an unbroken ribbon when whisk is lifted from bowl).
4. Reduce speed to low and with motor running, gradually pour in brown butter in a thin, continuous stream; beat until incorporated, about 2 minutes. Remove bowl from mixer and gently fold in 1½ cups (187 g) all-purpose flour with a rubber spatula, scraping down sides and bottom of bowl, until no large, dry streaks of flour remain. Scrape batter into prepared pan; smooth surface. Top with reserved hazelnuts and press in gently to adhere (surface will be smothered with nuts).
5. Bake blondie until edges are golden brown and center is just set, 35–45 minutes. Transfer pan to a wire rack and let blondie cool in pan. Using parchment paper to help, lift blondie out of pan and transfer to rack.
6. Meanwhile, combine 3 large egg whites and 1 cup (200 g) granulated sugar in the clean bowl of stand mixer and set over a saucepan of just simmering water (make sure bottom of bowl does not touch water). Heat, whisking constantly, until sugar is dissolved and an instant-read thermometer registers 170°.
7. Fit bowl onto mixer fitted with clean whisk attachment. Beat egg white mixture until thick and glossy and mixture is barely lukewarm, 8–10 minutes. Add 1 tsp. vanilla bean paste or vanilla extract and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt; fold in with a rubber spatula just to combine. Taste meringue and season with more kosher salt if needed
8. Peel away parchment paper from blondie and transfer to a heatproof platter. Using an offset spatula, spread meringue evenly over blondie, covering from edge to edge. Using a kitchen torch, toast meringue until deep golden brown. Sprinkle with flaky sea salt. Using a long knife and wiping off with a damp kitchen towel between cuts, slice blondie into 9 squares. Do Ahead: Blondie can be baked 24 hours ahead. Store, loosely covered, at room temperature.