Preheat your oven to 160 C/ 325 F. Line the bottom of a 6 inch round pan with parchment paper.
Whisk together the flour, baking powder and half of the sugar .
To this, add and whisk in the yolks, oil , juice, zest and water until smooth . Keep aside.
In another bowl , beat egg whites until foamy.
Then add the remaining half cup of sugar and beat until you get stiff peaks.
Gently whisk 1/4th of the whipped egg whites into the remaining batter. Similarly, whisk in the remaining egg whites in 3 or 4 additions.
Pour the batter into the prepared pan and bake for 30-35 minutes until top is brown and springs back when pressed gently.
Cool completely in pan . Then run a knife along the inside edge of pan and remove the cake carefully.
Serve as is , or with whipped cream or a fruity sauce!