Separate egg yolks and egg whites while they are still cold into two bowls. Place the egg yolks in one bowl and egg whites (along with the extra egg white) in another bowl. Cover it with plastic wrap, keep them aside and bring them to room temperature.
In a bowl, add the all-purpose flour, ½ cup of caster sugar, baking powder, salt, orange zest and mix it well.
Add orange juice, egg yolks, oil, and beat until a smooth batter is formed & set it aside.
In a separate bowl, add egg whites and beat until foamy. Add cream of tartar and mix it well. If you don’t have the cream of tartar, you can omit it or substitute it with an equal amount of lemon juice.
Add ¼ cup of caster sugar in batches and beat until stiff peak forms.
Now, gently fold the egg whites ( in 3-4 additions) into the prepared egg yolk batter with the spatula's help or wire whisk just until blended. Take care not to deflate the batter.
Grease the baking pan with butter or oil and pour the prepared cake batter.
Bake in preheated oven at 170 C for 45-50 minutes or until a skewer inserted into the centre of the cake comes out clean.
Remove the cake from the oven and let it cool down on the wire rack.
Serve with fresh fruit and whipped cream.