Special equipment: Two 17 1/2 x 11 1/2 inch jelly roll pans Instant read thermometer 14 x 4 inch piece of corregated cardboard wrapped in foil
Form the sheet of frozen yogurt: Line the bottom and sides of one of the pans with aluminum foil. If the frozen yogurt is rock hard, refrigerate it briefly until softened slightly; it must be scoopable but still very firm. (Note that frozen yogurt melts much more quickly than ice cream, so it must be very firm when you begin to work with it.) Distribute scoops of yogurt all over the pan. Working quickly and leaving about 4 inches at one end of the pan uncovered, spread the yogurt evenly in the pan by mashing the scoops with the back of a large spoon and pushing them together. Cover with foil and freeze at least several hours or until needed.
Make the cocoa souffle sheet: Position the rack in the lower third of the oven and preheat to 350 degrees F. Line the bottom of the second pan with a piece of parchment or wax paper.
In a large bowl, beat the egg yolks with vanilla and 1/2 cup of the sugar until pale and thick. Set aside.
In a separate mixing bowl, with clean dry beater, beat the egg whites and cream of tartar on medium speed until soft peaks form. Gradually sprinkle in the remaining sugar, continue to beat at high speed until stiff but not dry. Mix a quarter of the egg whites into the yolk mixture to lighten it. Scrape the remaining whites into the bowl and sift the cocoa over them. Fold until completely incorporated.
Spread the batter evenly in the pan and bake until the pastry springs back when gently pressed with fingertips, 12 to 15 minutes. Cool completely, in the pan on a wire rack.
Sprinkle a large sheet of foil with cocoa. Run a small knife or metal spatula around the edges of the cocoa pastry to detach it from the pan. Invert the cocoa pastry onto the foil sheet. Remove the pan and peel the paper pan liner from the cocoa pastry.
Assemble the roll: Remove the sheet of frozen yogurt from the freezer and remove the foil covering. Center and invert the yogurt sheet on top of the cocoa souffle sheet. Remove the pan and the foil. The long edges should be lined up and but there should be 2 inches of cocoa souffle exposed at each end.
Fold the exposed pastry over the yogurt at one end of the sheet. Continue to roll the pastry and yogurt up together, using the foil underneath as an aid as necessary. Do not worry if there is some initial cracking as you roll, however, if the yogurt is much too stiff to roll, wait a few minutes until it is pliable. Wrap the roll in foil and return to the freezer overnight or until hard.
Preheat the oven to 425 degrees F.
Make the meringue topping: Bring 1-inch of water to a simmer in a large saucepan. Combine the water, cream of tartar and sugar in a 6 to 8 cup stainless bowl. Whisk in the egg whites. Place thermometer near stove in a mug of very hot tap water. Set bowl in saucepan. Stir mixture briskly and constantly with a rubber spatula, scraping the sides and bottom often to avoid scrambling the egg white. After 1 1/2 minutes, remove the bowl from the skillet. Quickly insert thermometer, tilting bowl to cover stem by at least 2 inches. If less than 160 degrees F, rinse thermometer in simmering skillet water and return it to mug. Replace the bowl in skillet. Stir as before until temperature reaches 160 degrees F when bowl is removed. Beat on high speed until cool and stiff. Set aside.
Remove the frozen roll from the freezer and unwrap it. Place the roll on the foil wrapped cardboard. Use a metal spatula to cover the roll completely with a thick coating of meringue topping. Leave no cake exposed and be sure that the meringue touches the foiled cardboard all along the base of the roll. Use a fork to texture the meringue to resemble tree bark. Transfer the buche on its foiled base to a baking sheet. Sieve powdered sugar over the buche. Bake just until golden, 4 to 6 minutes, watching carefully. Slide a wide spatula or pancake turner under the foiled cardboard and transfer the buche to a serving platter. Serve immediately with Bittersweet Chocolate Sauce, if desired.
Combine the sugar and cocoa in a heavy bottomed medium saucepan. Whisk in just enough buttermilk to form a smooth paste. Whisk in the remaining buttermilk. Cook over medium heat until the mixture simmers and begins to boil, stirring constantly with a wooden spoon, scraping the sides and bottom of the pan. Boil gently for 3 minutes, stirring constantly to prevent burning. Remove from the heat and stir in the vanilla. Cool. Serve at room temperature or slightly warm. Sauce may be stored in the refrigerator up to one week.