Prep the Oven and Pan: Preheat your oven to 338°F / 170°C. Line the bottom of the cake pan with parchment paper. Do not grease the sides. The sponge needs to "climb" the walls of the pan to reach its full height. Alternatively, line the side with baking paper.
Sift the Flour/Starch: In a small bowl, whisk together the white rice flour and cornstarch. Sift them at least twice.
Whip the Eggs: In a large bowl, beat the egg with the sugar, salt and the vanilla extract. Use a hand or stand mixer on high speed for 4-5 minutes, until the mixture is pale yellow and has tripled in volume.
Folding the Flour: Sift the flour/starch mixture over the egg foam in three stages. Fold gently after each addition until no white streaks remain. Do not deflate the egg.
Incorporate the Oil: In a small bowl, take a small amount of the cake batter and mix in the sunflower oil until well combined. Gently fold the mixture back into the cake batter.
Baking: Pour the cake batter into the prepared pan. Bake for 40-45 minutes. The cake is done when the top is golden brown and springs back when lightly touched.
Cooling: Remove from the oven and immediately run a thin knife around the edge. Invert the cake onto a wire rack to cool completely.