Step 1

Preheat oven according to brownie packet instructions. Grease a 23cm springform cake pan and line base with baking paper. Prepare brownie mix according to instructions and spread into prepared pan.

Step 2

Bake according to instructions, removing 5 minutes before cooking time so it is slightly undercooked. Cool completely (the brownie will pull away from edge of pan). Grease sides of pan and line with the acetate around the brownie and about 5cm above the pan.

Step 3

Place chocolates in a heatproof bowl set over a saucepan of simmering water (don't let the bowl touch the water) and stir until melted and smooth. Cool slightly, then beat in egg yolks. Set aside.

Step 4

Soak gelatine in cold water for 5 minutes to soften. Place 200ml cream in a small saucepan over low heat and bring to a simmer. Squeeze excess water from gelatine and stir into cream until melted and combined. Set aside to cool, then beat into chocolate mixture.

Step 5

Whisk remaining 700ml cream to stiff peaks in a bowl and set aside. Place eggwhites in a stand mixer and whisk to stiff peaks. Add sugar, 1 tbs at a time, and beat until sugar is dissolved and mixture is glossy. Fold whipped cream into chocolate mixture, then, in 2 batches, fold in eggwhites. Pour over brownie base and chill overnight until set.

Step 6

Warm outside of pan with a hot cloth, then gently release cake from pan to serve. Cut into slices and serve.