Preheat oven to 350 degrees (175 C) and place an oven rack in the center of the oven.
Line the bottom of an 8-inch cake pan with parchment paper to fit (see how-to video above) and lightly brush the bottom and sides of the pan with oil or cooking spray.
Whisk the cornmeal, flour, baking powder and salt in a bowl and set aside.
Beat the eggs, egg whites and sugar in a heavy-duty stand mixer on medium-high speed 4-5 minutes, until pale and creamy. On low speed, mix in the oil. butter, yogurt, lemon zest and juice.
Stir in the dry ingredients until just blended. Pour the batter into the pan and bake 35 - 40 minutes, or until the top feels firm (not hard) and a toothpick inserted in the center of cake comes out clean. Cool in the pan 10 minutes before inverting onto a rack to cool - run a dull knife around the edge of the pan to loosen.
Sift powdered sugar over the cake and serve.
Serve the cake plain or with lightly sweetened fresh fruit.
The cake keeps well for several days at room temperature.