In a bowl, mix cherry jam and rum.
Unroll cold cake.
Spread with jam.
Cover with half the whipped cream.
Gently roll again.
Trim a 1 to 2 inch (2.5 to 5 cm) diagonal slice off one end of roll.
Set aside for use to simulate a tree knot.
With a spatula, spread a layer of mocha butter cream over cake, reserving 1/4 cup (60 mL) to cover knot.
Do not frost log ends.
Place knot on top of log.
Coat sides with remaining mocha butter cream.
With a spatula, mark log and knot with ridges to imitate bark.
With a spatula, spread remaining whipped cream over log ends and on top of knot.
Using a pastry bag with a very small nozzle, pipe swirls of chocolate onto whipped cream.
With a small knife, draw lines perpendicular to spiral.
Using a sieve, lightly dust cocoa over contour of log.
Decorate with holly, red currants and meringue mushrooms.