Heat oven to 180c. Line bottom only of 15x10x1-inch pan with waxed paper.
Line a 8 X 12" rectangular cake pan with parchment paper.
In a large mixing bowl, add in flour, cocoa powder, salt and baking soda. Sieve and set aside.
In another bowl, add egg yolk, sugar, vanilla extract, milk and oil together and whisk to mix well.
Add in the sifted flour mixture to the yolk mixture and stir to combine until fully incorporated, do not over mix. Set this chocolate-flour mixture aside.
In a clean and dry, non-plastic mixing bowl, whip egg whites on slow speed until frothy.
Increase speed to medium, once the whites become opaque and bubble size have tightened up, add sugar a little bit at a time.
Increase speed to med-high, whip until stiff peaks.
Gently fold 1/3 of the meringue into the chocolate-flour mixture to get it moving. Once incorporated, fold in the rest of the meringue. Do not stir. Use batter immediately.
Dab a bit of the batter onto the sides of the parchment paper to keep corners together. Dab a little more onto the paper and stick it to the pan to prevent the parchment paper from tipping over into the cake batter.
Pour into lined cake pan from one spot, spread out the batter evenly with an angled spatula.
Tap the pan on the counter to release large air pockets.
Bake for 14-16 min. at 175 degrees C until a skewer pulls out clean and cake springs back in the center.
While hot, carefully roll up cake and towel from narrow end. Cool completely on cooling rack.
Unroll cake carefully, and remove towel. Spread filling evenly over cake; roll up cake. Place cooling rack on sheet of waxed paper. Place cake roll on cooling rack; frost cake. Using fork, drag tines through frosting to look like log. Let stand 15 minutes. Decorate with xmas accessories.
Whip the cream with an electric mixer till stiff peak. Slowly add in the melted chocolate. Fold in using a spatula to mix evenly.
Place chopped chocolate into a metal bowl.
Heat cream on the stovetop until just simmering. Turn off the heat and immediately pour the warm cream over the chocolate.
Let the two sit for a few minutes before stirring. Stir until smooth. Cool for 15 mins before using.