Instructions

• Line an 8-by-8-inch square baking pan with parchment paper. Spread the 1/2 cup (175g) Nutella in a thin, even layer over the bottom of the pan. Freeze until the Nutella is solid, at least 20 minutes.
• While the Nutella freezes, brown the butter: melt the 3/4 cup (1 1/2 sticks/170g) unsalted butter in a light-colored saucepan over medium heat. Swirl the pan occasionally. The butter will foam, then the foam will subside as the milk solids sink and begin to turn light golden brown. Remove from heat as soon as the solids are a light golden brown and the butter smells nutty. Watch closely to avoid burning.
• Let the browned butter cool to lukewarm (so it is no longer hot to the touch).
• Preheat the oven to 350°F (177°C). If you have a second 8-by-8-inch pan, line it with parchment and set it aside. If you only have one pan, keep the Nutella-lined pan in the freezer on the parchment; when the Nutella is solid you will remove the Nutella layer from that parchment and set the parchment with Nutella back in the freezer while you line the pan with a fresh piece of parchment.
• In a large bowl or the bowl of a stand mixer fitted with the whisk attachment, combine 1 1/4 cups (250g) packed light brown sugar, 2 large eggs, 1 large egg yolk, 2 teaspoons vanilla extract, 1/2 teaspoon kosher salt, and 1 teaspoon baking powder. Beat on high speed for 2 to 3 minutes, until the mixture is lighter in color and thickened but not stiff.
• Reduce the mixer to low and slowly drizzle in the lukewarm browned butter until fully incorporated.
• Sprinkle the 2 cups (250g) all-purpose flour over the batter and mix on low speed until just incorporated. Scrape down the sides of the bowl as needed; do not overmix.
• Fold in 1/2 cup (75g) semisweet chocolate chunks or chips and 1/2 cup (65g) coarsely chopped hazelnuts, reserving a spoonful of each for the top.
• Spread half of the dough in an even layer in the prepared (clean) 8-by-8-inch pan.
• Remove the frozen Nutella from the freezer. If it is still in its parchment, gently lift the Nutella layer and set it, Nutella-side down, on top of the dough in the pan. If you used two pans, lift the Nutella layer out of its pan and place it on the dough in the second pan. Press gently so the Nutella sits flat and even.
• Spread the remaining dough in an even layer over the Nutella so it is fully covered. Sprinkle the reserved chocolate chunks and hazelnut pieces over the top, and finish with a light sprinkling of Maldon flake sea salt.
• Bake on the center rack for 35 to 40 minutes, until the top is shiny and set and the edges are starting to brown. (Blondies may still be moist inside; use the appearance of the top and edges rather than a toothpick as your guide.)
• Transfer the pan to a wire rack and let the blondies cool to room temperature.
• Lift the entire block of blondies from the pan using the parchment paper. Using a large chef’s knife, cut into 16 even squares, wiping the knife clean between cuts for neat slices.
• Store the blondies in an airtight container at room temperature for up to 3 days.