For the chocolate cake:

1. Preheat oven to 180°C (350°F). Grease and line a 32×22 cm rectangular baking tin, letting the parchment overhang the sides slightly.
2. Dissolve the instant coffee granules in the boiling water in a measuring jug. Set aside to cool.
3. In a large bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Stir in the sugar.
4. Pour in the buttermilk, vegetable oil, eggs, and vanilla bean extract. Whisk until evenly combined and glossy. Pour in the cooled coffee and whisk until smooth — the batter will be thin and runny. Pour into the prepared tin.
5. Bake for 30–32 minutes until risen and a skewer inserted into the middle comes out with a few moist crumbs. Remove from the oven and cool in the tin for 15 minutes, then carefully turn out onto a wire rack to cool completely before frosting.

For the frosting:

1. Beat the softened butter on medium-high speed until pale and fluffy, about 3–4 minutes.
2. Add the sifted powdered sugar and cocoa powder in two additions, beating well between each. Add the vanilla and a pinch of salt.
3. With the mixer running on low, drizzle in the cooled brewed coffee. Increase speed and beat until light and creamy. The coffee must be completely cold — if it is warm it can melt the butter. If the frosting looks slightly loose, refrigerate for 10 minutes then beat again.
4. In a separate cold bowl, whip the heavy cream to soft peaks. Gently fold the whipped cream through the buttercream in two additions until just combined — this is what gives the frosting its mousse-like lightness.
5. Spread the frosting thickly over the cooled cake. Finish with a generous pinch of fleur de sel or flaked sea salt over the top. Serve in generous slabs.