Preheat oven 375F. Grease large jelly roll pan (17"x12") with butter and fit with waxed paper and grease. Melt chocolate in double broiler. Mix egg whites, salt, cream of tartar and sugar, beat firm.
Add coffee and vanilla extract. Add melted chocolate to egg yolks. Stir in the whites in choco-yolk mix, fold with rubber spatula. Pour mix and bake at 375 for 10 min, then 350 for 15 min. Let cool.
Prepare filling. Whip cream until firm. Add vanilla extract, sugar, cocoa and coffee. Turn out cake onto clean dish towel. Spread filing and roll long side.
Prepare frosting. Coarsely chop chocolate. Bring cream and butter to boil over medium heat. Remove from heat and whisk chocolate until melted. Transfer to bowl and let cool at room temperature.
Assemblage. Place log on serving platter.
Spread frosting around the platter to create mud effect.
Roll marzipan in small cylinders for the mushroom stem and balls for the cap. Pinch the ball into cap and press gently onto stem. Dust with cocoa.
Arrange mushrooms and other things.
Dust confectioner sugar for snow effect.