Photograph: Tara Fisher
Preheat the oven to 180°C, fan 160°C, gas 4. Put the chocolate and olive oil in a bowl and set over a pan of simmering water. Stir until completely melted, then set aside. Grease a 22cm springform tin and line the base with nonstick baking paper.
In a large bowl and using an electric hand whisk (or an electric mixer), whisk together the egg yolks and sugar until the mixture is pale, thick and voluminous, this will take a good five minutes or so. Then gently stir in the melted chocolate mixture.
In a clean bowl and using clean whisks, beat the egg whites until they form stiff peaks. Gently fold the whites into the chocolate mixture. Set the tin on a baking tray and pour the chocolate mixture into it.
Bake for 30 minutes. Leave the cake to cool completely in the tin.
To serve, release the cake from the tin and transfer to a serving plate or board. Dust the top with cocoa and serve with berries and ice cream.
Get ahead: the cake can be made a day or so ahead, or can be frozen.
Kitchen secret: this sinks as it cools in the tin – don't panic, it's quite normal!