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1
Form the sheet of frozen yogurt: Line the bottom and sides of one of the pans with aluminum foil.
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2
If the frozen yogurt is rock hard, refrigerate it briefly until softened slightly; it must be scoopable but still very firm.
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3
(Note that frozen yogurt melts much more quickly than ice cream, so it must be very firm when you begin to work with it.)
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4
Distribute scoops of yogurt all over the pan.
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5
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.
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6
Cover with foil and freeze at least several hours or until needed.
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7
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.
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8
In a large bowl, beat the egg yolks with vanilla and 1/2 cup of the sugar until pale and thick.
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9
Set aside.
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10
In a separate mixing bowl, with clean dry beater, beat the egg whites and cream of tartar on medium speed until soft peaks form.
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11
Gradually sprinkle in the remaining sugar, continue to beat at high speed until stiff but not dry.
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12
Mix a quarter of the egg whites into the yolk mixture to lighten it.
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13
Scrape the remaining whites into the bowl and sift the cocoa over them.
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14
Fold until completely incorporated.
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15
Spread the batter evenly in the pan and bake until the pastry springs back when gently pressed with fingertips, 12 to 15 minutes.
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16
Cool completely, in the pan on a wire rack.
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17
Sprinkle a large sheet of foil with cocoa.
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18
Run a small knife or metal spatula around the edges of the cocoa pastry to detach it from the pan.
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19
Invert the cocoa pastry onto the foil sheet.
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20
Remove the pan and peel the paper pan liner from the cocoa pastry.
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21
Assemble the roll: Remove the sheet of frozen yogurt from the freezer and remove the foil covering.
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22
Center and invert the yogurt sheet on top of the cocoa souffle sheet.
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23
Remove the pan and the foil.
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24
The long edges should be lined up and but there should be 2 inches of cocoa souffle exposed at each end.
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25
Fold the exposed pastry over the yogurt at one end of the sheet.
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26
Continue to roll the pastry and yogurt up together, using the foil underneath as an aid as necessary.
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27
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.
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28
Wrap the roll in foil and return to the freezer overnight or until hard.
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29
Preheat the oven to 425 degrees F.
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30
Make the meringue topping: Bring 1-inch of water to a simmer in a large saucepan.
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31
Combine the water, cream of tartar and sugar in a 6 to 8 cup stainless bowl.
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32
Whisk in the egg whites.
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33
Place thermometer near stove in a mug of very hot tap water.
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34
Set bowl in saucepan.
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35
Stir mixture briskly and constantly with a rubber spatula, scraping the sides and bottom often to avoid scrambling the egg white.
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36
After 1 1/2 minutes, remove the bowl from the skillet.
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37
Quickly insert thermometer, tilting bowl to cover stem by at least 2 inches.
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38
If less than 160 degrees F, rinse thermometer in simmering skillet water and return it to mug.
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39
Replace the bowl in skillet.
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40
Stir as before until temperature reaches 160 degrees F when bowl is removed.
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41
Beat on high speed until cool and stiff.
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42
Set aside.
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43
Remove the frozen roll from the freezer and unwrap it.
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44
Place the roll on the foil wrapped cardboard.
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45
Use a metal spatula to cover the roll completely with a thick coating of meringue topping.
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46
Leave no cake exposed and be sure that the meringue touches the foiled cardboard all along the base of the roll.
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47
Use a fork to texture the meringue to resemble tree bark.
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48
Transfer the buche on its foiled base to a baking sheet.
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49
Sieve powdered sugar over the buche.
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50
Bake just until golden, 4 to 6 minutes, watching carefully.
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51
Slide a wide spatula or pancake turner under the foiled cardboard and transfer the buche to a serving platter.
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52
Serve immediately with Bittersweet Chocolate Sauce, if desired.
