01 - Preheat oven to 325°F. Line the bottom of an 8-inch round cake pan with parchment paper. Do not grease the sides.
02 - Whisk together flour, cornstarch, baking powder, and salt in a bowl. Set aside.
03 - Whisk 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 - Beat egg whites with an electric mixer on medium speed until soft peaks form. Gradually add 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 batter into the prepared pan and smooth the top with a spatula.
08 - Bake for 40-45 minutes, or until set and a toothpick inserted in the center comes out clean.
09 - Let cool in the pan for 10 minutes, then carefully run a knife around the edge and invert onto a wire rack. Peel off parchment and cool completely.
10 - Dust with powdered sugar and serve with berries if desired.