Preheat the oven to 350℉/180℃ and line the bottom of 9" cake or springform pan with parchment paper.
In a bowl, whisk together the flour, baking powder, and salt and set aside.
In the bowl of a stand mixer fitted with the whisk attachment, add the egg whites and lemon juice (or vinegar) and whisk on medium speed until foamy.
With the mixer running, slowly pour in half of the sugar (½ cup, 100g) and increase the speed to medium high speed until stiff, glossy peaks form. The meringue will stand up as a stiff peak on the tip of the whisk.
Use a rubber spatula to scrape out the meringue into a separate bowl.
Add the egg yolks and remaining sugar (½ cup, 100g) to the bowl of the stand mixer and mix on medium speed with the whisk attachment until pale in color, fluffy and doubled in volume, and ribbons form.
With the mixer running on medium speed, slowly pour in oil then scrape down the sides. Turn the mixer back on to medium and slowly pour in the water and vanilla. Mix for another minute.
Use a fine mesh sieve to sift ⅓ of the flour mixture into the egg yolk mixture and fold it in with a rubber spatula. Then, add ⅓ of the egg whites and fold it in until no streaks remain.
Repeat the process with the remaining flour mixture and egg whites, taking care to get the spatula to the very bottom of the bowl (the flour will sink to the bottom and get stuck). Take care not to over mix, but very gently fold.
Pour the batter into the prepared pan, and bake for 25-30 minutes until golden and a toothpick inserted into the center comes out clean. Cool on a wire rack for 5 minutes, then run an offset spatula or knife around the sides and remove the cake from the pan to finish cooling.