First, make the chocolate mousse. Melt the butter and chocolate over a low heat, stirring constantly to not let the chocolate catch. Once smooth, remove from the heat, pour into a big bowl and allow to cool slightly
Separate the eggs and add the egg yolks to the chocolate mixture, stirring vigorously after each egg yolk is added, so that the yolks don’t get hot
Place the egg whites into a separate bowl, add a pinch of salt and beat with an electric whisk until really stiff
Add a spoonful of egg white to the chocolate mixture and stir so that the chocolate becomes a slightly more runny. Add the remaining egg whites in three parts, each time gently folding them into the chocolate mixture with a wooden spoon or silicone spatula. You want it to remain nicely fluffy, so don’t beat the mixture
Once all of the egg whites are incorporated, pour the mousse into six small bowls, cover with cling film and place in the fridge for 3 hours or overnight
Preheat the oven to 190°C/gas mark 5
While the mousse is setting, make the shortbread. In a bowl, beat the butter and the sugar together with an electric whisk until smooth. Scrape out the vanilla bean paste from the vanilla pod and add to the butter. Mix again to combine
Add the flour and mix with the electric whisk until it looks crumbly. Use your hands to knead the mix until a dough forms. This will be very crumbly at first, but give it a bit of time and the dough will come together
Place the dough onto a floured surface and roll out until it is roughly 1cm thick. Cut into rectangles and prick a few times with a fork to give the shortbread its traditional look
Place the shortbread on a baking tray lined with baking paper, sprinkle with caster sugar and bake for 15-20 minutes, until golden-brown. Allow to cool on a wire rack before serving with the chocolate mousse
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