Preheat oven to 350 degrees F and prepare a 17- x 12-inch jelly roll pan as follows: grease, line bottom with parchment or foil, extending slightly over sides, and then grease again and flour.
In a mixing bowl, beat 1/4 cup sugar and the egg yolks for 5 minutes or until light and fluffy. Add the chocolate and beat until incorporated, scarping down the sides of the bowl.
In a large mixing bowl, beat the egg whites until foamy, add the cream of tartar, and beat until soft peaks form when the beater is raised. Gradually beat in the remaining 2 tablespoons sugar, beating until still peaks form when the beater is raised slowly.
With a large balloon whisk, slotted skimmer, or rubber spatula, fold 1/4 of the whites into the chocolate mixture to lighten it. Then gently fold in remaining egg whites.
Pour into the prepared pan, spreading evenly with a spatula, and bake 16 minutes. The cake will have puffed and lost its shine and will spring back when lightly pressed with a finger.
Meanwhile, wet a clean dish towel and wring it out well. Remove the cake from the oven and leave it in the pan. Dust with the cocoa powder and cover immediately with the damp towel. Allow the cake to cool while making the ganache and whipped cream.