Place the egg yolks, 1/4 cup sugar and orange rind in a large bowl. Beat with an electric mixer at high speed until the mixture is thick and lemon-coloured, about 10 minutes. Add the vanilla and beat to combine.
Place the egg whites in a separate large bowl and beat with clean beaters at high speed until foamy. Add the cream of tartar and beat until soft peaks form. Add the remaining sugar, 2 tablespoons at a time, and beat until the sugar dissolves and stiff, glossy peaks form.
Sift 1/3 cup flour over the egg whites and gently fold in with a whisk just until the flour is no longer visible. Repeat three times until all the flour is used. Spoon one-third of the egg white mixture into the yolk mixture and gently fold in.
Spoon heaped tablespoons of the yellow and white mixtures by turns into a 28- or 30-cm ring tin. Swirl a thin spatula or knife through the mixture to give a marbled effect. Lightly swirl the top of the cake, too. Bake until the cake springs back when lightly touched, about 35 minutes.
Leaving the cake in the tin, invert it and place over the neck of a bottle to cool completely. (Cooling the cake in the tin the right way up will cause it to sink.) Run a knife around the cake to loosen it. Turn it out onto a plate and dust with icing sugar.
When beating egg whites, always use a clean bowl and clean beaters. Even a trace of fat or egg yolk can cause the egg whites to collapse or prevent them from forming stiff peaks. Cream of tartar added to the egg whites gives the cake a fine grain and helps to keep it from pulling away from the side of the tin.