5. Assemble the cake.
Unroll cake; remove second sheet of parchment. Spread about 2 cups frosting over cake. You will likely have some filling left over — use as a decadent pancake topping, cupcake filling, etc! Starting at one long side, roll up the Yule Log jelly-roll style. Place cake, seam side down, on platter of your choosing. Starting 1 inch in from the end of the cake, turn knife on sharp diagonal and cut to remove one 3-inch-long triangular piece of cake. Spread 1 tablespoon frosting on cut side of cake piece. Attach frosted side of cake piece to side of cake near end to make a branch.
6. Make the frosting.
Using an electric mixer, cream together the margarine and the shortening. Add the cocoa powder and incorporate well. Add the confectioners sugar in about 1/2 cup batches and beat well, adding a little non-dairy milk after each addition. Add the vanilla and beat 2-3 minutes. This frosting can be used immediately on the Yule Log.
7. Frost the cake.
Cover the cake with frosting. The amount that I gave in the directions gave me enough frosting to exactly cover the Yule Log. The Yule Log will look pretty and natural even if you just frost the cake by using long, wide streaks of frosting, but I liked how the log looked using the tines of a fork to create texture, so I followed the same technique.
8. Decorate the cake.
Sift some powdered sugar over the Yule Log for a snowy look. Also, marzipan mushrooms, leaves, fruits, and other creatures are cheerful additions and fun for kids to create. Non-edible figurines (like gnomes and fairies) would also be cute. The mushrooms on my cake took less than two minutes to make: Roll a piece of marzipan into a ball. This will be your mushroom cap, so take an appropriate-sized amount. Use the tip of your thumb to make an indentation on the bottom of the ball. Turn it around and around with your two index fingers (kind of like you are shaping a mini pizza) and shape it into a mushroom cap. Take a smaller piece of marzipan for the mushroom stem and roll it to desired length, making a base for the bottom. Attach stem inside mushroom cap.
Serve your Yule Log and enjoy! Makes about 12 slices.