For the brown butter streusel:

1. Place the butter in a small saucepan over medium heat. Stir often until golden and melted. Increase heat to medium-high and cook, swirling occasionally, until the butter turns amber and smells nutty — brown solids will just begin to form at the base. Remove from heat and set aside to cool slightly.
2. Place the roughly crushed graham cracker crumbs in a small bowl. Stir in the sugar and fleur de sel. Pour in the brown butter and mix until the crumbs are evenly moistened. Set aside.

For the brownie:

1. Preheat oven to 175°C. Grease and line a 32 × 22 cm rectangular baking tin with parchment paper, letting it overhang the sides. Lightly grease the parchment.
2. Place the butter in a medium saucepan over medium heat and melt. Pour into a large mixing bowl. Add the granulated sugar and whisk to combine. Add the eggs one at a time, whisking until smooth and glossy. Whisk in the vanilla extract and dissolved espresso.
3. Sift over the cocoa powder, flour, and salt. Fold in with a rubber spatula until smooth and combined.
4. Pour the batter into the prepared tin and spread to the edges. Scatter the brown butter streusel over the top. Bake for 16–19 minutes, until the edges are firm, the middle is puffed and set but slightly soft, and the streusel is deeply toasted. Cool completely in the tin.

For the fleur de sel ganache:

1. Place the finely chopped dark chocolate in a heatproof bowl. Heat the cream in a saucepan until just beginning to boil. Pour over the chocolate and let sit for 1 minute. Stir with a rubber spatula until completely smooth. Stir in the butter and fleur de sel until incorporated.
2. Pour the ganache over the cooled brownie while it is still in the tin. Use an offset spatula to spread it evenly to the edges. Refrigerate for 30–45 minutes until the ganache is set and firm.

For the meringue:

1. In the bowl of a stand mixer, stir the egg whites and sugar together. Set over a saucepan of simmering water — do not let the base of the bowl touch the water. Heat, whisking occasionally, until the mixture reaches 70°C on a candy thermometer, scraping down to prevent sugar crystallisation.
2. Transfer to the stand mixer fitted with the whisk attachment. Beat on medium-high until stiff, glossy peaks form and the bowl is cool to the touch, about 6 minutes. Add the vanilla bean seeds and beat until combined.
3. Turn the brownie out onto a serving plate. Spread the meringue over the set ganache in swoops and swirls to create peaks. Torch the meringue with a kitchen blow torch until evenly toasted, keeping the torch away from the ganache layer. Sprinkle with extra fleur de sel before slicing. Serve at room temperature. Keeps up to 3 days in an airtight container in a cool, dark place.