Preparation Steps

• Position a rack in the center of the oven and preheat to 350°F. Line an 8x8-inch baking pan with parchment, letting the paper hang over two opposite sides for easy lifting; lightly butter the parchment.
• Set a heatproof bowl over (but not touching) a simmering pot of water. Add the chopped dark chocolate and cubed butter to the bowl and stir occasionally until melted and smooth. Remove from the heat and set aside to cool slightly while you prepare the eggs.
• In a large metal or glass bowl, whisk together the eggs, granulated sugar, dark brown sugar, espresso powder (if using), and vanilla. Place the bowl over the same pot of simmering water for 1–2 minutes, whisking constantly, until the mixture is warm and the sugars feel dissolved.
• Remove the egg mixture from the heat and use an electric mixer on medium-high to beat until the mixture is pale, thick and slightly ribbon-like, about 3–4 minutes.
• Slowly drizzle the melted chocolate-butter into the whipped egg mixture, mixing on low speed until combined and glossy. Be careful to pour slowly so the eggs don't deflate.
• Sift the flour and cocoa powder together, then fold them gently into the chocolate mixture with a rubber spatula until no streaks remain and the batter is shiny and thick.
• Spread the batter evenly into the prepared pan and smooth the top. Bake at 350°F for 26–30 minutes — the edges should be set and the center will still have a slight jiggle and a glossy skin on top. Avoid overbaking to keep the brownies fudgy.
• Remove the pan to a wire rack and let the brownies cool in the pan for 30 minutes. After 30 minutes, you can transfer the whole pan (still on the parchment) to the refrigerator for an extra 15 minutes to firm up if you prefer a cleaner spread.
• While the brownies cool, loosen the Nutella by microwave-heating it in a small microwave-safe bowl for 8–12 seconds (or place the closed jar in hot water). It should be spreadable but not hot — overheating makes it runny and harder to set.
• Once the brownie surface is cool to the touch, lift it from the pan using the parchment. Spread the warmed Nutella evenly over the top, arrange the sliced strawberries across the surface, then drizzle any remaining Nutella and finish with a light sprinkle of flaky sea salt.
• If the Nutella softens the topping, chill the finished pan for 10 minutes to set everything, then slice into 9 squares and serve. Store leftovers refrigerated in an airtight container for up to 3 days.