Butter one (1) 10 x 15 inch jelly-roll pan, then line it with parchment paper and butter it again.
Set a rack in the middle of the oven and preheat to 400 degrees F.
Half-fill a medium saucepan with water and bring it to a boil over high heat. Lower the heat so the water is simmering.
Whisk the eggs, egg yolks, salt and sugar together in the bowl of a heavy-duty mixer. Place over the pan of simmering water and whisk gently until the mixture is just lukewarm, about 100 degrees if you have a thermometer (or test with your finger – it should be warm to the touch).
Attach the bowl to the mixer and, with the whisk attachment, whip on medium-high speed until the egg mixture is cooled (touch the outside of the bowl to tell) and tripled in volume. The egg foam will be thick and will form a slowly dissolving ribbon falling back onto the bowl of whipped eggs when the whisk is lifted.
While the eggs are whipping, stir together the flour, cocoa and cornstarch.
Sift one-third of the flour mixture over the beaten eggs. Use a rubber spatula to fold in the flour mixture, making sure to scrape all the way to the bottom of the bowl on every pass through the batter to prevent the flour mixture from accumulating there and making lumps. Repeat with another third of the flour mixture and finally with the remainder.
Scrape the batter into the prepared pan and smooth the top.
Bake the genoise for about 10 to 12 minutes. Make sure the cake doesn’t overbake and become too dry or it will not roll properly.
While the cake is baking, begin making the buttercream.
Once the cake is done (a tester will come out clean and if you press the cake lightly it will spring back), remove it from the oven and let it cool on a rack.