Once your cake has cooled, carefully unroll it and spread the ermine filling into an even layer. Gently roll the cake back up into a log.
Wrap tightly with plastic wrap and place it into the refrigerator to chill and firm up. The cake can chill for a minimum of 8 hours, but overnight works great as well. This allows all the elements to have adequate time to chill, set, and get ready for decorating your yule log cake.
The next day, when you are ready to assemble your yule log, gather all your ingredients onto the counter. You will want to have your serving platter out to place the yule log on to decorate it directly on the serving platter.
Unwrap your yule log and carefully slice off a 1½ to 2-inch end piece at a slight angle if you want your yule log to have a little piece of a side branch. If you want to keep it simple, you can just place your cake roll onto the serving plate. Set aside.
To assemble your mushrooms, carefully remove a mushroom cap and turn it over. On the underside of the mushroom cap, using the point of a chopstick or something similar, make a little hole that the point of the stem will fit into by twisting the point of the stick into the bottom. Handle the mushrooms very gently.
Take a little bit of the firm chocolate ganache and spread it onto the bottom of the mushroom cap, then place a stem into the ganache to hold the two pieces together. Place the assembled mushrooms onto a plate. Repeat until you have assembled enough mushrooms to use for decorating your yule log. Once you have made all your mushrooms, you can then whip your chocolate ganache to make the frosting.
Once you have whipped your chocolate ganache frosting, you will spread some onto the angled side of the piece you cut off of your cake roll to adhere it to the side of your yule log.
Frost the remaining cake with a small offset spatula. Try to be mindful to frost the cake in long strokes to resemble the bark of a tree. You do not need to try to frost the bottom/underside of your cake as it is delicate and may break apart your yule log. That is why you are frosting and decorating directly on your serving platter.
Place your candied rosemary sprigs around the bottom and sides of your yule log, then place the candied cranberries on top.
Place your meringue mushrooms onto the yule log and on the platter. You can use a small amount of leftover ganache frosting to adhere the mushroom to the platter to prevent them from moving or falling over.
Dust your yule log and serving platter with a light dusting of powdered sugar to make it look like snowfall.