Preheat oven to 320F (160F) and have a 10-inch tube with feet pan ready.
Sift the flour and baking powder in a medium bowl and mix in one cup (200g) of the sugar.
Mix the egg yolks, orange zest, orange juice, and vegetable oil, in a medium bowl. Then, mix the flour mixture with the egg mixture.
Whip the egg whites for about one minute until frothy, then gradually add the remaining sugar and whip to a medium peak. This is very important for the success of the cake, so stay close by.
Fold the meringue into the egg yolk mixture in three parts.
Pour the cake batter into an ungreased tube pan and bake for 50-55 minutes.
Remove the cake from the oven and immediately turn the cake upside down.
Notes
Medium stiff meringue is the key to the cake's success. That means the meringue is opaque and stable, creating white streaks but forming a soft ribbon when you dip the whisk into it and turn it upside down.
Do not use an electric mixer to mix the egg yolks; too much air might cause the cake to collapse. Instead, use a hand whisk.
To maintain the cake's light texture, fold the egg whites gently into the batter using a soft rubber spatula. Allow the spatula to reach the bottom of the bowl, then lift it gently.
When zesting the orange, avoid the white pith as it can be bitter.
Use a serrated knife or an angel food comb to slice the cake without deflating it.