01 - Preheat your oven to 325°F. Line the bottom of an 8-inch round cake pan with parchment paper. Do not grease the sides.
02 - In a bowl, whisk together flour, cornstarch, baking powder, and salt. Set aside.
03 - In a large bowl, whisk the egg yolks with 1/2 cup sugar until pale and creamy. Add oil, yogurt, vanilla extract, and lemon zest; mix until smooth.
04 - Gently fold the dry ingredients into the wet mixture until just combined.
05 - In a separate clean bowl, beat the egg whites with an electric mixer on medium speed until soft peaks form. Gradually add the remaining 1/4 cup sugar and beat until stiff peaks form.
06 - Gently fold one-third of the beaten egg whites into the batter to lighten it, then fold in the remaining egg whites in two more additions. Do not overmix.
07 - Pour the batter into the prepared pan and smooth the top 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.
09 - Remove from the oven. Let cool in the pan for 10 minutes, then carefully run a knife around the edge and invert onto a wire rack. Peel off the parchment and let cool completely.
10 - Dust with powdered sugar and serve with berries if desired.