Make the cake: Preheat oven to 425 degrees. Butter the jelly-roll pan and line it with wax paper, leaving a 2-inch overhang on each of the short ends. Butter the paper and dust it with flour, shaking out the excess.
In the large bowl of the upright mixer beat the egg yolks until smooth. Add 6 tablespoons of the granulated sugar, a little at a time, and beat the batter at medium speed for 3-4 minutes, or until creamy and light. Beat in the framboise. In another bowl with the hand-held mixer beat the egg whites with 1/8 teaspoon of the salt until frothy. Add the cream of tartar and beat the whites until they hold soft peaks. Add the remaining 2 tablespoons sugar, a little at a time, and beat the whites until they hold stiff peaks. Fold the whites into the batter gently but thoroughly. Sift the flour, the 1/4 cup cocoa powder, and the remaining 1/8 teaspoon salt over the batter, one third at a time, and fold in each addition until batter is smooth.
Pour the batter into the jelly-roll pan, spread it evenly with a spatula, and bake the cake in the middle of the oven for 8-10 minutes, or until it pulls away from the sides of the pan and a cake tester inserted in the center comes out clean.
Dust the cake lightly with cocoa powder, cover it with wax paper, and let it cool to warm. Pull the edges of the wax paper away from the pan, invert the cake onto a baking sheet and carefully peel off the paper. Dust the cake lightly with the remaining cocoa powder, cover it with wax paper, and roll it up. Let the cake cool completely.
Make the chocolate butter cream: In the bowl of the upright mixer beat the egg yolks until light and creamy. In a small heavy saucepan combine the sugar with the water. Bring the mixture to a boil and cook it over moderate heat, stirring and washing down any sugar crystals clinging to the sides of the pan with the brush dipped in cold water, until the syrup reaches the soft-ball stage, or the candy thermometer registers 238 degrees. With the mixer running, add the hot syrup to the yolks in a stream, beating, and beat the mixture until completely cool. Beat in the butter and the chocolate, a little at a time. Pour the buttercream into a bowl, beat in the framboise, and chill the buttercream, covered, until the cake is ready to be frosted.
To assemble the cake: Cut a 2-inch piece diagonally from each end of the cake and set the pieces aside. Transfer the cake to a serving tray and arrange the reserved end pieces on top of the cake to stimulate sawed-off branches. Beat the chilled buttercream until smooth and with a spatula spread it over the cake. Gently pull the tines of a fork lengthwise over the buttercream to simulate bark and garnish the log with the meringue mushrooms and chocolate leaves. Force the green almond paste through a medium sieve and with the tip of a small knife attach it to the log around the mushrooms to simulate moss. Gently dust the log with sifted confectioners' sugar, simulating snow. Let the cake come to room temperature before serving. Cut the cake with a sharp serrated knife.
In a heavy saucepan combine the 1 cup sugar with the water and bring the mixture to a boil over low heat, washing down any sugar crystals clinging to the sides of the pan with the brush dipped in cold water until the sugar is dissolved. Boil the syrup until it reaches the soft-ball stage, or until the candy thermometer registers 240 degrees.
While the syrup is cooking, in a bowl with the mixer beat the egg whites with the salt until foamy. Add the cream of tartar and beat the whites for 30 seconds. Sprinkle in the remaining 1 tablespoon sugar and beat the meringue for 10 minutes, or until cool.
Preheat the oven to 200 degrees. Line baking sheets with the parchment paper. Fill the pastry bag fitted with the plain tip with the meringue and pipe out 66 mounds, each about 1-inch in diameter, 1-inch apart onto the baking sheets. Sift a bit of the cocoa over each cap, if desired, to simulate sand. Holding the pastry bag straight up, pipe out 66 medium-wide lengths onto the baking sheets, to resemble mushroom stems. Bake the meringues in the middle of the oven for 2 hours. Remove the baking sheet from the oven and with your finger tip push in the underside of each mushroom cap. Return the meringues to the oven and bake them for 30 minutes more. Turn off the oven and let the meringues stand in the oven overnight.