Heat the oven to 180°C / 350°F / Gas 4. Line the base of a 20cm/8 in. springform cake tin with baking paper, then grease the base and the sides with butter.

In a mixing bowl, whisk together the flour, sugar, cocoa and baking powder, ensuring there are no lumps - sift the mixture if necessary. Pour in the hot water, oil and vinegar, stir until smooth, then scrape into the prepared cake tin, smoothing the surface. Bake in the preheated oven for 10-12 minutes, until a skewer inserted in the centre of the cake comes out clean. Allow to cool completely in the tin.

When the cake is cold, remove it from the tin, remove the baking paper from the bottom and place the cake on the serving plate or stand that you plan to use. Replace the outside of the springform around the cake, lining with cake acetate or baking paper if using (see note).

To make the mousse, soak the gelatine leaves in a bowl of cold water for at least 5 minutes. Meanwhile, melt the chocolate in a large heat-proof bowl set over a pot of simmering water. When melted, remove from the heat and allow to cool slightly.

In a small saucepan, combine 50ml of the cream with the sugar, and gently heat, stirring until the sugar is dissolved. Remove from the heat, then squeeze as much liquid out of the gelatine sheets as you can and stir into the hot cream. Pour the mixture into the melted chocolate and stir until smooth.

In a mixing bowl, whip the rest of the cream until stiff peaks form - for this amount of cream it is best to use an electric beater. Scoop around a quarter of the cream into the bowl with the chocolate and stir to combine. At this point, the cream and chocolate will separate and you may think that everything has gone wrong. Don’t panic! Keep stirring until the mixture is smooth, it will come back together again.

Using a spatula or a large spoon, fold the remaining cream under the chocolate mixture. Be gentle but thorough, you want to keep as much air in the mixture as possible, but also to ensure it is evenly mixed. Spoon the mousse on top of the cake, smooth the top and give it a little shake to even it out, then refrigerate for at least 2 hours.

To make the ganache, put the chocolate into a small heatproof bowl. Heat the cream in a small saucepan until nearly boiling, then pour over the chocolate. Add the oil and allow to sit for 5 minutes. After 5 minutes, use a spoon (not a whisk) to beat the mixture until smooth. Allow to cool a further 5 minutes, then pour over the chilled cake, gently tilting to spread it evenly across the surface.

Chill the cake for at least 2-3 more hours until set, better overnight. In hot weather or if your fridge is very full this is an important step! (see note)

Before serving, remove the springform edge, carefully remove the cake acetate or baking paper (if using). If you like, top with some fresh berries. Use a sharp knife dipped in warm water to cut.