Cake: Preheat oven to 170 degrees C. Grease and flour two 23cm cake tins.
Sift together the flour, baking powder and cream of tartar. Set aside.
In a medium bowl beat together the egg yolks and sugar until thick and pale. Stir in the lemon zest and lemon juice.
In a large glass or metal mixing bowl beat egg whites and salt until soft peaks form. Fold whites into the egg yolk mixture alternately with the flour mixture. Spread evenly into the prepared cake tins.
Bake for 25 to 30 minutes in the preheated oven or until a toothpick inserted into the cake comes out clean. Let layers cool in their tins for 15 minutes before inverting onto wire racks to cool completely.
Lemon Curd Filling: In the top of a double boiler combine the sugar, egg yolks, lemon zest and lemon juice. Cook over high heat, stirring constantly until the sugar is dissolved and mixture thickens. Remove from heat and stir in the butter.
Coconut Icing: In a medium bowl cream the butter until light and fluffy. Gradually add the icing sugar and mix in the orange zest, orange juice, lemon zest and lemon juice. Finally stir in coconut.
To Assemble: Using a long serrated knife, slice the cake layers in half horizontally.
Cool the lemon curd filling to room temperature then use to fill between the layers of the cake.
Use the Coconut icing to ice the outside of the filled cake.