Preheat oven to 150 degree/275 -300 F. Grease 20 cm ring pan; line base with baking paper. Combine butter, sugar, chocolate and the water in medium saucepan; stir over heat until sugar dissolves. Remove from heat; Let it cool for 10-15 mins. Stir in rum, nuts and fruit. Add sifted dry ingredients and egg; stir until combined. Spoon mixture into prepared pan.
Bake in slow oven about 1 hour. Cool cake in pan.
Just before serving, combine cooled extra chocolate and sour cream in small bowl; stir until smooth. Turn cake onto serving plate, top-side up; spread chocolate mixture over top of cake.
Notes
I made only half of the above quantity. As usual I didn't do the topping. I used a 6 inch round cake tin and my cake was ready in 45-50 mins. You can use dry fruits of your choice. I used cashew nuts, dates, mixed dry fruits. I dusted the dry fruits with 1 tbsp of flour before adding it to the batter, this prevents the fruits from sinking to the bottom of the cake. I used dark chocolate with 50 % cocoa. I used soft light brown sugar. I brushed the top of the warm cake with 2 tbsp extra rum. I used Captain Morgan rum in the cake batter and I used Bacardi rum for brushing the top of the cake. I stored the cake at room temperature for more than a week. If you want to store longer, refrigerate it.