Preheat oven to 160°C (140°C fan, gas mark 2-3), grease a deep 20cm (8in) round cake tin and line the base with greaseproof paper.
Put all the ingredients for the butter sponge in a mixing bowl and beat with an electric mixture on low speed until just combined. Increase the speed to medium and beat for 1-2 minutes until the mix is smooth and paler in colour.
Divide the mix equally between three bowls. Tint one quantity with a few drops of pink food colouring, if using, then gently stir in the raspberries. Sift the cocoa over the second bowl and stir in with the chocolate. Leave the third quantity plain.
Alternating the mixtures, drop heaped spoonfuls into the prepared tin. Shake the tin to distribute the mixtures evenly, and gently tap it on the work surface to release any air bubbles. Bake for an hour and 10 minutes until a skewer inserted in the centre comes out clean. Leave the cake in the tin to cool, before turning onto a rack. Carefully peel off the lining paper and turn the cake over to cool completely. Dust with icing sugar before serving.