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.

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).

Place the two types of chocolate 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 (this will be extremely thick and curdled looking, but don’t worry as the added cream will smooth it out again). Set aside.

Soak gelatine in cold water for 5 minutes to soften. Place 200ml (6.7 fl oz) of the 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 a little, then beat the cream into the chocolate mixture.

Whisk remaining 700ml (23.6 fl oz) cream to soft peaks in a bowl and set aside. Place egg whites in a stand mixer and whisk to soft peaks. Add sugar, 1 tablespoon at a time, and beat until sugar is dissolved (don’t over-beat or mixture will be too difficult to fold in).

Fold whipped cream into chocolate mixture. Then, in 2 batches, fold in egg whites (given the volume of mixture, you may need to separate it out into two bowls, as I did).

Lightly spray one side of the acetate sheet/s with cooking oil (to prevent the mousse from sticking to it), then line the pan with acetate with the oiled side inwards (the acetate should fit right down to the bottom of the pan, between the pan and the brownie). Make sure the acetate is at least 5cm above the top of the pan.

Pour the mousse mixture over brownie base (you might find that if there’s a wide gap between the pan and the brownie, the mousse mix will settle into that gap – don’t worry, it just means you won’t see much of the brownie base from the outside when its unmoulded). Level the top with an off-set spatula, and place the cake pan in the fridge to chill overnight until set.

Gently release the pan from around the dessert and carefully remove the acetate. If the mousse sticks a little to the acetate, simply place an off-set spatula into warm water, wipe it dry, then smooth around the cake afterwards.

Very carefully, remove the cake pan base and baking paper (I used a large dinner-plate sized round spatula to do this), and place cake on a serving platter. Arrange some of the raspberries on top of the cake to decorate.

Cut into slices (to ensure clean slices, dunk the knife in warm water, dry with a kitchen towel and cut). As the cake is rather top-heavy, I found it easier to lay each piece on its side to serve.

Serve remaining raspberries alongside each served slice, together with a dollop of softly whipped cream. Put any leftovers into fridge – it keeps beautifully for a further day.