Put the chocolate, butter and salt into a heatproof bowl and set it over a saucepan of simmering water, ensuring the bottom of the bowl doesn’t touch the water. Stir occasionally until the chocolate and butter have both melted, then remove from the heat and set aside to cool.

Put the egg whites and sugar into the bowl of a stand mixer and, using the balloon whisk attachment, whisk them slowly before increasing the speed for a couple of minutes until the whites form stiff peaks. Gently scrape the mixture into a bowl.

Clean the bowl of the stand mixer and, using the balloon whisk attachment again, whip the cream very gently on a low speed, just so that it thickens slightly. This should take no more than 30–60 seconds – watch it carefully. However, if you are making the mousse to chill overnight and serve the following day, only use half of the cream for the mousse, using the other half fresh the next day to serve with the mousse, as whipped cream is generally better made fresh.

Next, very gently fold the egg whites into the chocolate, one spoonful at a time. Your main aim is to keep the air in the egg whites, as this is what will make the mousse light and fluffy so work delicately. Once all the egg whites are incorporated, gently fold in half the whipped cream, making sure not to overwork the mixture. Set the rest of the whipped cream aside for when you are ready to serve the mousse.

Refrigerate the mousse for at least 1 hour, or longer if possible. You could make this the night before you serve it, so that it’s completely ready to go if you’re hosting and want to get ahead.

Once set, serve in bowls with a spoonful of whipped cream, a sprinkle of sea salt and a drizzle of extra virgin olive oil, if using.