Preheat oven to 180°C or 160°C fan. Lightly grease and line base of a 23cm springform pan.
Using an electric beater, beat egg yolks and sugar until pale and thick. Sift flour with baking powder and fold into egg mixture along with melted butter and orange rind and juice. Using an electric beater or whisk, beat eggwhites until soft peaks form. Fold into cake mixture.
Spoon mixture into prepared pan and bake for 50 minutes. Cool for 2 minutes then transfer to a wire rack and cool for 5 minutes, until cool enough to handle but still warm. Wash and dry cake pan.
Meanwhile, make syrup by combining orange juice, rum or brandy and sugar in a jug. Stir briskly to dissolve sugar. Return cake to clean pan. Spoon syrup over warm cake and set aside to cool completely. Transfer to a serving plate.
Make chocolate topping by placing chocolate and coffee in a heatproof bowl. Place over a pan of simmering water on low heat. Stir continuously until chocolate melts. Remove from heat and whisk in sour cream, until smooth and glossy. Cool for 5 minutes before spreading over cake.