Set the oven to 170°C. Grease and line a spring form cake tin (20cm). Sprinkle base of cake tin with a little brown sugar and place plum halves around the tin. Beat butter and sugar until creamed. Add eggs and beat one at a time. Fold in flour and coconut then add milk and stir until a smooth batter. Spoon the batter into the tin over the plum halves. Bake until risen and golden and a skewer comes out clean - about 50 minutes to 1 hour depending on your oven. You may want to cover the cake tin half way through the cooking time to prevent the top from catching. Serve with extra plums and some organic yoghurt.