In a heatproof mixing bowl set over a pan of gently simmering water, place the dark chocolate and 1 tbsp of butter and allow them to melt for 2-3 minutes. Once smooth and melted, remove from the heat and set to one side to cool slightly.
In a separate clean bowl, whisk 3 large egg whites with a pinch of salt until they form stiff peaks. Beat 3 egg yolks into the cooled chocolate mixture until smooth. Carefully fold the whites into the melted chocolate mixture, taking care not to knock too much air out of the mixture.
Once incorporated, transfer the chocolate mousse to a piping bag and pipe into 4 glass tumblers so that the mixture comes a third of the way up the glasses. Chill in the fridge whilst you prepare the raspberry mousse.
In a saucepan, combine 350g of the raspberries and 50g of sugar and cook on a medium heat for 5 minutes until the fruit softens. Pass this mixture through a sieve so that you have at least 200ml of raspberry coulis.
In a separate clean bowl, whisk 3 large egg whites with a pinch of salt and 50g of the sugar until stiff peaks form. In another bowl, combine the yogurt and 200ml of raspberry coulis until smooth. Soak the gelatine in a jug with 4 tbsp of boiling water until dissolved. Add this to the raspberry mixture and then whisk in a third of the egg white mixture. Gently fold in the remaining egg whites and and transfer to a clean piping bag. Pipe the mousse on top of the chocolate mousse in the glasses and return to the fridge to set for 3-4 hours.
Meanwhile, prepare the shortbread by creaming together the unsalted butter with the icing sugar. Beat in the egg yolk and a pinch of salt. Mix together the flour and cocoa powder and slowly combine with the butter and sugar mixture until well combined. Wrap this mixture in clingfilm and chill for at least an hour.
Pre-heat the oven to 180°C before rolling out on a lightly floured surface to 1cm in thickness.
Use a heart-shaped cookie cutter to cut out biscuits and arrange them spaced out on a lined and greased baking sheet. Bake for 12-15 minutes until just cooked. Remove and allow the shortbreads to cool on the sheet before removing to a wire rack to cool completely.
At serving point, arrange three raspberries in each glass on top of the mousse and garnish each with a sprig of mint. Serve the shortbread biscuits on the side.
*This pudding is very rich and indulgent so it shouldn't be eaten too often. But it's perfect for those special occasions.