01 - Preheat oven to 325°F (160°C). Line the bottom of an 8-inch (20 cm) round cake pan with parchment paper. Do not grease the sides of the pan.
02 - In a medium bowl, whisk together flour, cornstarch, baking powder, and salt until well blended. Set aside.
03 - In a large bowl, whisk egg yolks with 1/2 cup (100 g) sugar until pale and creamy, about 2-3 minutes. Add oil, yogurt, vanilla extract, and lemon zest. Mix until smooth and fully incorporated.
04 - Gently fold the dry ingredients into the wet mixture using a spatula. Mix until just combined, taking care not to overmix.
05 - In a separate clean bowl, beat egg whites with an electric mixer on medium speed until soft peaks form. Gradually add remaining 1/4 cup (50 g) sugar and continue beating until stiff, glossy peaks form.
06 - Gently fold one-third of the beaten egg whites into the batter to lighten the mixture. Fold in the remaining egg whites in two more additions, being careful not to deflate the mixture.
07 - Pour the batter into the prepared pan. Smooth the top evenly with a spatula.
08 - Bake for 40-45 minutes, or until the cake is set and a toothpick inserted in the center comes out clean. The top should be lightly golden.
09 - Remove from the oven and let cool in the pan for 10 minutes. Carefully run a knife around the edge to loosen, then invert onto a wire rack. Peel off the parchment paper and allow to cool completely.
10 - Dust the cooled cake generously with powdered sugar. Serve with fresh berries if desired. For best texture, serve chilled.