Butter a jelly roll pan, line with parchment paper, butter the paper and sprinkle with flour, tapping to remove excess flour. Preheat oven to 350 degrees.
In a medium bowl, whisk the egg yolks with 2 tablespoons of the sugar. Meanwhile, place the egg whites in the bowl of an electric mixer and beat until soft peaks form. Turn the machine up to high, and beat in the remaining 3 tablespoons of sugar, continue to beat until shiny but not stiff. Add the flour to the yolk/sugar mixture and whisk to combine. Add l/4 of the egg whites to the yolk mixture, whisking to lighten the mixture. Then return the yolk mixture to the egg whites, and fold gently. Scrape into the prepared pan and even the top by running a long offset spatula across the surface. Bake until cake is golden brown and springs back when lightly touched, l2 to l5 minutes. Cool on a rack. Set aside.
For the Hazelnut Mousse:
Place cooled hazelnuts on a kitchen towel and rub together to remove skins. Place the skinned hazelnuts in a food processor fitted with a steel blade. Process for l minute. Add the hazelnut oil through the feeder tube and process until a paste forms.
Melt chocolate in a medium bowl placed over simmering water, or melt in a microwave oven. Add the melted chocolate to the hazelnut paste and set aside.
Whip the heavy cream until stiff peaks form. Fold in the hazelnut/chocolate mixture. Keep covered in the refrigerator until needed.
To assemble the cake roll:
Trim the edges of the sponge cake. Place the Hazelnut mousse on the cake layer. Spread the mixture evenly over the cake, leaving a l inch space along the long side. Using the parchment paper to aid you, carefully roll up the cake into a thick log, enclosing the mousse. Leave the wrapped, rolled-up cake on the sheet pan and refrigerate until filling is firm, at least 2 hours.
For the Chocolate Frosting:
Melt the chocolate. In a small pot, heat the coffee and cocoa. Add the sour cream. In the bowl of a mixer, beat the butter and sugar until light and creamy. Add the chocolate mixture and combine completely. Transfer to a medium bowl and set aside until of spreading consistency.;
For the Meringue Mushrooms:
Preheat oven to 200 degrees. Line l or 2 baking trays with parchment paper and set aside. Fit a large pastry bag with a #4 or #5 plain tip and set aside.
Beat the egg whites on medium speed until frothy. Turn speed to high, slowly pour in half the sugar, add vanilla and continue to beat for l minute. Slowly pour in the remaining sugar and beat until the whites are very thick and shiny, about 3 to 4 minutes longer. Immediately spoon half the mixture into the prepared pastry bag. (Meringue will begin to break down if not used soon after it is ready.) Hold the pastry bag upright over the prepared baking trays, squeeze out 26 to 30 mushroom caps, making different sizes, placed about l inch apart. The caps can be different sizes, since real mushroom caps are never identical.
Again, holding the bag upright over the other tray, lifting the bag as you squeeze out the meringue, form 26 to 30 stems, each l/2 inch to l inch high, placed about l inch apart.
Place the trays in the oven and bake until the meringues are firm to the touch and can be lifted easily from the trays, about l l/2 hours. The meringues should be dry, but should not take on any color. Remove the trays to a cool dry spot. When cool, with a small knife, trim any pointed tips on the caps and stems, if desired.
Melt the chocolate in a small bowl, and holding the caps upside down, spread a little chocolate on each one. Attach a stem and leave them upside down until the chocolate hardens. When ready, turn mushroom cap side up, and dust lightly with sifted cocoa.
Set aside until ready to use for decoration.
To decorate and serve:
Remove pan with cake from refrigerator and place wrapped cake on a firm surface. Carefully unwrap cake. Using a serrated knife, from one end cut a 2-inch piece of cake on the diagonal. Set the cut piece aside.
Cut out a piece of cardboard the length and width of the cake. Cover the cardboard with foil and place the cake on it. Spread a little frosting on the bottom of the small cut piece and set on top of the cake roll. Using a small spatula, spread the frosting over the entire cake as well as the small piece atop the cake. With a wooden skewer or the point of a small knife, make furrows to simulate bark along the sides and top of the cake.
Arrange Christmas greens, plastic holly, and Meringue Mushrooms around the Yule Log. Sift powdered sugar over the whole thing to simulate snow. Fresh cranberries can be scattered around for added color.