Whisk together the flour, cornstarch, salt, baking powder and baking soda and set aside.
Using a hand held mixer, beat the butter for about a minute till creamy. Beat in the sugar for another couple of minutes till pale and fluffy. Beat in the orange and lemon zest and eggs, one at a time.
Add about half the orange juice and half the flour mixture and beat on low speed till just combined. Add remaining orange juice and flour mixture and beat again till just combined and creamy.
Do not over mix. Transfer to a well-greased and floured 7” cake pan. Bake at 180C (350F) for 35 to 45 minutes or till the cake is done and skewer inserted in the centre comes out clean. Let it cool in the pan for about 5 to 10 minutes, then unmould. Cool on a rack till completely cool. Then top with icing.
For the Icing :
Beat together all the icing ingredients till smooth and fluffy. Adjust to desired consistency by adding more icing sugar or orange juice as desired. Spread on top of the cake with a spatula. Decorate as desired.