Chocolate Sponge Cake: Preheat oven to 350 degrees F (180 degrees C) and place the oven rack in the center of the oven.  Butter, or spray with Pam, a 17 inch (43 cm) by 12 inch (30 cm) jelly roll pan.  Line the pan with parchment paper and then butter and flour the paper (or spray with Baker's Joy).  Set aside.

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). 

Meanwhile melt the chocolate in a stainless steel bowl set over a saucepan of simmering water.   Set aside.

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 until this mixture is 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 melted chocolate 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 remaining 2 tablespoons (28 grams) of 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 deflate.   Spread the batter evenly into the prepared pan with an offset spatula.  Bake until the cake is puffed, has lost its shine, and springs back when gently pressed, about 15-17 minutes.  Remove from oven and place on wire rack to cool.  Cover the cake with a clean, slightly damp towel.

Chocolate Whipped Cream:  In a large mixing bowl place the whipping cream, vanilla extract, sugar, and cocoa powder and stir to combine.  Cover and chill the bowl and beaters in the refrigerator for at least one hour so the cocoa powder has time to dissolve.  Beat the mixture until stiff peaks form.

Once the cake has cooled, spread with the chocolate whipped cream (set 2 tablespoons aside) and then gently roll the cake, peeling off the parchment paper as you roll.  The sponge cake will crack, and, in fact, this makes it look more like a real log.  Trim one end of the cake at an angle and set it aside.  Then place the sponge cake, seam side down, on your serving platter.  Take the slice of reserved cake and, using the reserved whipped cream, attach it to the side of the sponge cake (to look like the end of a branch).  Cover with plastic wrap and chill until serving time.  Just before serving remove the cake from the refrigerator, dust with confectioners (powdered or icing) sugar (to resemble snow), and decorate with meringue mushrooms (if desired).  You can also decorate the Yule Log with miniature pine cones and pine needles. 

Adapted from The Cake Bible by Rose Levy Beranbaum