This grand dessert is a descendant of the medieval subtlety—food disguised to look like something else. Here, chocolate cake masquerades as the traditional yule log for a long winter’s night. Although making the log look realistic can be challenging, even a simple approach is great fun to make and delightful to serve on a holiday evening.
Cake: Preheat oven to 350°F (177°C). Line a jelly roll pan with greased parchment paper. Lay a clean kitchen towel flat on a work surface; dust with confectioners’ sugar and set aside. Place egg whites in a small bowl and set aside. Place egg yolks in a large bowl and beat until fluffy, about 3 minutes. Gradually add half of sugar, beating until thick and lighter in color, about 5 minutes. Gradually add flour, cocoa and salt; beat until well blended. Beat egg whites on medium speed until foamy; add cream of tartar and beat until soft peaks form. Gradually sprinkle in remaining sugar, 1 T (12 g) at a time, beating on high until stiff peaks form. Transfer ¼ of egg whites into yolks, stirring just to combine. Gently fold in remaining egg whites until blended. Spread batter evenly in prepared pan. Bake 12 minutes or until cake springs back when lightly pressed in center (do not overbake). Cool only 3 minutes, loosen sides with a small knife and invert onto prepared kitchen towel. Peel off parchment paper; loosely roll cake up in towel beginning with a short side. Let cool.
Filling: Place cream in a large bowl; beat until it begins to thicken. Add confectioners’ sugar and coffee granules; continue beating until stiff peaks form. Cover; chill.
Frosting: Place all frosting ingredients in a large bowl and beat with an electric mixer until smooth.
Meringue Mushrooms: Preheat oven to 200°F (93°C). Line a baking sheet with parchment paper. Fit a pastry bag with a large plain round pastry tip (#6); set aside. Place egg whites in a medium bowl and beat at medium speed until foamy. Add cream of tartar; continue beating until soft peaks form. Gradually beat in sugar, then increase speed to high beating until stiff and glossy and fill piping bag. To make mushroom caps: pipe half of mixture onto prepared baking sheet into flattened balls in a variety of sizes ½ inch (13 mm) to 1½ inches (38 mm) across, smoothing tops with dampened fingers. For stems, pipe meringue into a variety of cone shapes 1 to 2 inches (50 mm) high. Reserve leftover meringue. Bake about 1 hour or until firm to touch. Remove from oven and, using a small knife, puncture a depression into bottom of each mushroom cap. Dab a bit of leftover meringue inside depression and “glue” the small ends of stems into caps. Bake an additional 20 minutes. Let cool and store airtight up to 2 weeks.
Frost Cake: When cake is completely cool, unroll and spread filling to within ½ inch (13 mm) of edge; re-roll. Place on serving platter (or foil-covered cardboard if freezing) and frost cake. Use a fork to make shallow squiggly lines down length of “log.” Chill until ready to serve or freeze.
Assembly: Dust tops of mushrooms with cocoa powder and decorate “log” with them; sift on confectioners’ sugar “snow.”
Note: For convenience, the cake can be made up to 10 days in advance and frozen. Just wait until the frosting has hardened, then wrap the cake well in plastic wrap and freeze. To serve, remove wrap, place on a serving tray, decorate with mushrooms and confectioners’ sugar, and let thaw for 1 hour.