Preheat the oven to 180°C (350°F) Gas 4.

In a food processor, grind half of the toasted hazelnuts and pulse until well ground. Chop the remaining hazelnuts into small pieces by hand.

Whisk the egg whites until light foam peaks form, then add the sugar, cocoa powder and vanilla and fold until smooth. Fold in all the hazelnuts. Pour into the prepared cake pan. Bake in the preheated oven until done – depending on your oven, this can take 25–35 minutes. Remove from the oven and leave to cool.

Soften the gelatine leaves in cold water for approx.10 minutes. Put the whipping cream and vanilla into a large bowl and whip until stiff. Set aside in the fridge.

Meanwhile, blend the strawberries in a blender until smooth. Pass through a sieve/strainer – you should end up with around 350g strawberry purée. Put the sugar and purée in a saucepan over a low heat.

Squeeze the gelatine leaves to remove any excess water, then add to the warm strawberry mixture. Whisk to combine. Remove from the heat and leave to cool.

Add ¼ of the cold whipped cream to the strawberry purée and mix well, then pour the strawberry mix into the remaining cream and fold carefully. It is important the mixture is not hot. Pour into a bowl or piping bag and leave to chill for at least 2 hours or, ideally, overnight. To assemble, put the hazelnut base on a plate, pipe the mousse on top and finish with berries and edible petals.