Chop chocolate into very small pieces and put in a medium bowl. Heat heavy cream to just before boiling. Add to chocolate and leave for a couple of minutes without disturbing. Stir well with a spatula until completely mixed. Let it sit at least 3-4 hours to set, preferably overnight.

Preheat oven to 350F (175C). Line an 11" (28cm) square baking pan with parchment paper.

Divide eggs into whites and yolks. Sift cake flour and cocoa together. Melt butter. Set all aside.

Beat yolks and 1/3 of sugar until a light color and very thick.

In a different bowl, whip egg whites. When the eggs are forming into meringue, start adding the rest of the sugar in 3 parts. Whip until almost forming stiff peaks.

In the bowl of beaten yolks, add 1/3 of the whipped egg whites, and mix in carefully without deflating the foam. Add this mixture to the rest of the meringue, and carefully mix in the same way.

Add flour mixture, sifting into the egg mixture, and carefully mix. Finally add melted butter and mix again.

Pour the batter into the prepared baking pan, spread evenly, and bake for 10-12 minutes. Take out from the oven, cool in the pan completely, covered with plastic.

In a bowl, put 1/2 of the buttercream and 2-3 Tbsp of ganache and mix well.

In a different bowl, put the rest of the buttercream and ganache and mix well.

If you like, cut out pieces of the cake with round cookie cutters to make branch stumps. Fill the holes with the frosting, and place round cake pieces on top.

Then spread the frosting (the darker cream) over the surface of the log except the surface of the ends and any branch stumps. Cover the surface of the ends of the log and of any branch stumps with filling (the lighter color cream). With a fork, make lines like the surface of trees (bark) and circles on the ends and stumps. Sprinkle cocoa powder. Decorate as you like.