Preheat the oven to 160°C (fan). Line a 25 cm cake tin with non stick baking paper.
Melt the cacao and butter by placing them in a large heatproof bowl set over a pan of simmering water.
Meanwhile, beat the eggs with the sugar and the vanilla extract until pale yellow and fluffy.
Pour the cacao mixture into the eggs and beat to combine.
Stand the lined tin in a roasting tray and fill the tin with the mousse mixture.
Pour enough boiling hot water into the roasting tray to come halfway up the side of the tin. Bake for 30 minutes or until set and leave to cool in the tin.
Willie’s Tip: In Venezuela- or anywhere hot and full of living things- the mousse cake would have to be kept in the fridge. Then, to turn it out, we’d run the dish over a gas flame or heat the sides with a blowtorch.