On Christmas Eve or Day, you will assemble your buche de Noel to serve!
Take the buttercream from the refrigerator and place in a bain-marie to warm up just until softened, and then whip well until smooth and spreadable.
Brush a liqueur, such as coffee or orange, on the cake; but do not saturate. (You can also use simple syrup, which is equal parts of sugar and water brought to a boil and allowed to cool, or poached pear syrup.)
Spread a thin layer of the buttercream on the cake, and then roll the cake from long end to long end of the pan. If the cake breaks, continue to roll carefully–you can cover any cracks with icing.
Lay the roll seam-side down on a platter, and ice completely with buttercream. Cut with a serrated knife on a diagonal about 3-4 inches from one end and stick the end piece on top (cut side up). Ice the cut ends. Run a fork through the buttercream to resemble bark, or just leave the rough spatula marks.
[If you are serving the same day, continue; but if you are serving the following day, stop here and cover with plastic wrap. Save any leftover icing for touch-ups.]
Decorate your gateau any way you would like, and finish with a sprinkling of fresh-fallen snow (sifted powdered sugar). Remember, it doesn’t have to be perfect; in fact, the most endearing yule logs look as though children made them (and even better if they actually did!).