01 - Preheat oven to 325°F. Line bottom of 8-inch round cake pan with parchment paper. Leave sides ungreased.
02 - Whisk flour, sugar, baking powder, and salt in medium bowl until uniform.
03 - Whisk egg yolks, Greek yogurt, oil, vanilla, and lemon zest in large bowl until smooth.
04 - Add dry ingredients to wet mixture. Stir just until combined, avoiding overmixing.
05 - Beat egg whites with electric mixer in clean bowl until stiff peaks form.
06 - Fold one-third of beaten egg whites into batter to lighten. Gently fold in remaining whites, preserving air pockets.
07 - Transfer batter to prepared pan. Smooth top with spatula.
08 - Bake 35-40 minutes until golden and toothpick inserted in center comes out clean.
09 - Let cool in pan 10 minutes. Run knife around edges, invert onto wire rack. Cool completely.
10 - Dust with powdered sugar if desired. Slice and serve.