Preheat the oven to 180C/350F/gas mark 4
Use a 23cm/9 inch square cake tin.
Line the base of the cake tin and grease well. Beat the butter and sugar until creamy. Add the sour cream and vanilla essence, and beat well. Sift the dry ingredients together, then sift again over the butter cream and fold in, alternately with the buttermilk.
Peel the orange, removing as much of the white pith as possible; chop very finely and mix in a bowl with the chopped nuts and sultanas. With a slotted spoon, spoon out one-quarter to one-third of this mixture and mix it into the cake mixture, folding lightly. To the remaining orange-nuts-sultana mixture, add the coconut, and set aside until after baking.
Spoon the cake mixture into the cake tin, and bake for 40 to 45 minutes. Remove from the oven and allow to stand for 5 minutes.
Preheat the grill. Carefully spread the reserved orange-nuts-sultanas mixture over the top of the cake, and place under the grill to toast lightly. Remove from heat and allow to stand for a further 5 minutes. Then carefully turn out onto a cake rack which has been covered with greaseproof paper. Reverse the cake to top side uppermost and allow to cool.