01 - Preheat the 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 medium bowl, whisk together the flour, sugar, baking powder, and salt until well incorporated.
03 - In a large bowl, whisk together the egg yolks, Greek yogurt, vegetable oil, vanilla extract, and lemon zest until smooth and uniform.
04 - Add the dry ingredients to the wet mixture and stir gently until just combined. Avoid overmixing to prevent toughness.
05 - In a clean bowl, beat the egg whites with an electric mixer until stiff peaks form. The whites should hold their shape when the beaters are lifted.
06 - Gently fold one-third of the beaten egg whites into the batter to lighten the mixture. Then carefully fold in the remaining egg whites, using a rubber spatula and retaining as much air as possible.
07 - Pour the batter into the prepared cake pan and smooth the top with an offset spatula.
08 - Bake for 35 to 40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
09 - Let the cake cool in the pan for 10 minutes. Run a knife around the edges to loosen, then invert onto a wire rack. Allow to cool completely before serving.
10 - Dust generously with powdered sugar before serving, if desired. Slice and enjoy as a light dessert or teatime treat.