Preheat oven to 350 degrees F (180 degrees C) and place the oven rack in the center of the oven. Butter, or spray with a non stick vegetable spray, a 17 by 12 inch (43 x 30 cm) sheet pan. Line the pan with parchment paper and then butter and flour the paper (or spray with vegetable/flour spray.
While the eggs are still cold, separate the eggs, placing the whites in one bowl and the yolks in another. Cover with plastic wrap and bring to room temperature before using (takes about 30 minutes).
In the bowl of your electric mixer (or with a hand mixer) place the egg yolks and 1/4 cup (50 grams) of sugar and beat, on high speed, until this mixture is thick, light and fluffy (about five minutes). (When you slowly raise the beaters, the batter will fall back into the bowl in a slow ribbon.) Beat in the vanilla extract. Add the coco powder and beat only to combine. Set aside while you beat the egg whites.
In a clean mixing bowl, with the whisk attachment beat the egg whites until foamy. Add the cream of tartar and beat at medium-high speed until soft peaks form. Gradually beat in the sugar until stiff peaks form.
Gently fold a small amount of the egg whites into the egg yolk mixture using a rubber spatula or whisk to lighten the batter. Fold in the remaining whites just until incorporated. (Don’t over mix or the batter will