To make the buttercream: Whisk the egg whites and sugar together in the bowl of an electric mixer. Set the bowl over simmering water and whisk gently until the sugar is dissolved and the egg whites are hot. Attach the bowl to the mixer and whip with the whisk on medium speed until cooled. Switch to the paddle and beat in the softened butter and continue beating until the buttercream is smooth. Dissolve the instant coffee in the liquor and beat into the buttercream.

Turn the genoise layer over and peel away the paper. Invert onto a fresh piece of paper. Spread the layer with half the buttercream. Use the paper to help you roll the cake into a tight cylinder Transfer to baking sheet and refrigerate for at least 30 minutes or until set. Reserve the remaining buttercream for the outside of the buche.

To make the marzipan: Combine the almond paste and 1 cup of the sugar in the bowl of the electric mixer and beat with the paddle attachment on low speed until the sugar is almost absorbed. Add the remaining 1 cup sugar and mix until the mixture resembles fine crumbs. Add half the corn syrup, then continue mixing until a bit of the marzipan holds together when squeezed, adding additional corn syrup a little at a time, as necessary; the marzipan in the bowl will still appear crumbly. Transfer the marzipan to a work surface and knead until smooth.

To make marzipan mushrooms: Roll 1/3 of the marzipan into a 6-inch long cylinder and cut into 1-inch lengths. Roll half the lengths into balls. Press the remaining cylindrical lengths (stems) into the balls (caps) to make mushrooms. Smudge with cocoa powder. To make holly leaves: Knead green color into 1/2 the remaining marzipan and roll it into a long cylinder. Flatten with the back of a spoon, then loosen it from the surface with a spatula. Cut into diamonds to make leaves, or use a cutter.

To make holly berries: Knead red color into a tiny piece of marzipan. Roll into tiny balls.

To make pine cones, knead cocoa powder into the remaining marzipan. Divide in half and form into 2 cone shapes. Slash the sides of cones with the points of a pair of scissors.

Unwrap the cake. Trim the ends on the diagonal, starting the cuts about 2 inches away from each end. Position the larger cut piece on the buche about 2/3 across the top. Cover the buche with the reserved buttercream, making sure to curve around the protruding stump. Streak the buttercream with a fork or decorating comb to resemble bark. Transfer the buche to a platter and decorate with the marzipan. Sprinkle the platter and buche sparingly with confectioners' sugar "snow."

Storage: Keep at cool room temperature. Cover leftovers loosely and keep at room temperature.