Line the base and sides of a 20cm/8in spring-form cake tin with greaseproof paper and set the oven to 180C/350F/Gas 4.
Melt the chocolate and butter in a bowl over a pan of simmering water.
In a separate bowl, whisk the yolks with 2 tbsp of the sugar. Stir in the melted chocolate and butter and mix well. Beat the egg whites with the remaining sugar until very stiff, quickly fold into the chocolate mix in two batches and pour the mix in the cake tin.
Place on the middle shelf of the oven and bake for 20 minutes.
To make the sauce, tip all the ingredients in a bowl, stir and then blend until smooth. For an extra smooth sauce simply pass through a sieve.
To serve, place two or three spoonfuls of sauce on the centre of a plate. Set a wedge of chocolate cake on top. Decorate with a few fresh raspberries and a sprig of mint.