Heat oven to 180C/160C fan/gas 4. Grease and line the base of 2 x 20cm round, loose-bottomed cake tins. Put the butter and chocolate in a heatproof mixing bowl, set over a pan of barely simmering water and allow to melt slowly, stirring occasionally.
When the butter and chocolate have melted, remove from the heat and allow to cool for 1-2 mins, then beat in the eggs and sugar with an electric whisk. Fold in the flour and raspberries.
Gently pour the mixture into the tins and bake for 20-25 mins or until golden brown and a skewer inserted into the centre comes out clean (the raspberries may leave a residue on the skewer, so don’t be fooled by their juiciness). Remove the cakes from the oven and allow to cool in the tins for 10 mins before turning out onto a wire rack.
To make the ganache, put the chocolate in a heatproof bowl with 100ml of the cream, set over a pan of barely simmering water. Stir until the chocolate has melted into the cream and you are left with a smooth, glossy ganache. Leave the ganache to cool to room temperature, then beat in the rest of the cream.
When the cakes have cooled, sandwich them together with the chocolate ganache. Dust with icing sugar just before serving.