1. Grease and line the base and sides of a 20-cm (8-in) spring-release cake tin. Melt 90g (3oz) of the butter in a saucepan, remove from the heat and then mix in the crushed shortbread. Press into the base of the tin. Chill until required.
2. Break the chocolate into pieces and place in a small heatproof bowl. Add the remaining butter and stand the bowl over a pan of gently simmering water. Allow to melt, then remove from the water and stir in the sour cream and rum if using.
3. Whip the double cream to form soft peaks and fold into the chocolate cream. Spoon over the biscuit base and smooth the top. Chill for at least 6 hours or overnight.
4. When ready to serve, remove from the tin and place on a serving plate. Top with the berries and finish off with white chocolate decorations of your choice. Cut into slices to serve.
This dessert will also work with milk or continental plain chocolate.