Setup a double boiler with a pot large enough to hold your mixing bowl then bring the water to a simmer. Move your oven rack to the middle position and preheat to 400 degrees Prep an 18″ x 13″ jelly roll pan by lining it with parchment paper, then butter the parchment paper.
Put the eggs, yolks, sugar and salt in a metal mixing bowl and whisk to combine. Put the bowl on your double boiler and whisk, heating until the mixture reaches 100 degrees F (luke warm). Mount the bowl on a mixer and beat on high with the whisk attachment for 5 minutes. The volume will triple and pale yellow ribbons of egg will flow off the whisk when they’re ready.
Sift together the flour and cornstarch. When the egg mixture is ready, add 1/3 of the flour mixture into the eggs and gently fold together the flour and eggs. Repeat twice more, folding between each addition until you don’t see any more clumps of flour. Pour the batter into the prepared baking sheet, and smooth off the top (a pastry knife or a clean plastic ruler works great for this). Bake for 7-8 minutes or until a toothpick comes out clean. Let the genoise cool on a rack.