Preheat oven to 160°C. Grease a 24cm springform cake pan and line the base and side with baking paper.
Place chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (don’t let the bowl touch the water). Stir until melted and smooth. Set aside to cool.
Vigorously whisk the eggs and sugars together in a bowl until well combined and sugar has partially dissolved. Whisk in hazelnut meal, cocoa, espresso and cooled chocolate mixture. Pour into prepared pan. Place pan on a baking tray and bake for 50 minutes until edges are set but centre is still a little bit wobbly. Cool to room temperature, then chill until cold. Cut into slices to serve.
