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 uniformly blended.
03 - Whisk egg yolks, yogurt, oil, vanilla, and lemon zest in large bowl until completely smooth.
04 - Add dry ingredients to wet mixture; stir gently until just combined. Avoid overmixing.
05 - Beat egg whites in clean bowl with electric mixer until stiff peaks form.
06 - Gently fold one-third of beaten whites into batter to lighten. Fold in remaining whites, maintaining maximum air volume.
07 - Pour batter into prepared pan; smooth surface evenly.
08 - Bake 35-40 minutes until golden brown and toothpick inserted center emerges clean.
09 - Cool in pan 10 minutes. Run knife around edges, invert onto wire rack. Cool completely.
10 - Dust with powdered sugar if desired. Serve immediately or store in airtight container.