Dissolve instant coffee granules in water. Set aside.
Whisk egg yolks and half of the castor sugar until light and fluffy. The mixture should hold a trail of the whisk for a few seconds. Beat in coffee mixture and corn oil. Lightly mix in self-raising flour.
In a separate clean bowl, whisk the egg whites, cream of tartar and remaining sugar until fluffy and stiff.
Gently fold egg yolk mixture into the egg whites until well blended. Turn the mixture into an ungreased 20cm chiffon cake pan.
Bake in preheated oven at 180°C for about 30-35 minutes or until cake is golden brown and well risen.
Invert the pan and let the cake cool completely. Ease the cake away from the sides of the pan and around the base with a palette knife.