For the sponge cake:

1. Preheat the oven to 320°F (160°C). Line a 14×10 inch (35×25 cm) baking sheet with parchment paper. 
2. Whisk together the yolks and sugar in a mixing bowl, until pale and fluffy. Add in the orange zests and sift in the flour over the mixture. Whisk until incorporated.
3. In a separate bowl, beat the whites with a pinch of salt, until stiff peaks form. Using a rubber spatula, gently fold in the egg whites until completely combined. Avoid over-mixing and deflating those whites. Batter will be very light.
4. Spread the batter into the prepared pan, flattening the top with a rubber spatula. Bake for 20 minutes or until baked through. Do NOT over-bake.
5. Peel off the parchment paper underneath the cake. If needed, you can trim the edges with a large knife for a neat cut on all four sides of the sponge cake.** Then, immediately roll up the cake tightly starting on the narrow end, and cover tightly with a damp towel to prevent from drying out, so it doesn’t crack.

For the chocolate filling:

1. Place the softened butter (not melted!) in a mixing bowl and stir.
2. Melt the chocolate in a water bath (preferred method) or in a small saucepan placed over very low heat. Stir until smooth, and cool off for a few minutes. Add the softened butter, confectioners’ sugar, and combine until smooth. Place in the refrigerator for 15 minutes or longer (the chocolate ganache should be thick enough to spread over the cake, but not too thick either).
3. Remove the damp towel, then slowly and gently unroll the cake. Spread about half of the chocolate ganache on top, leaving a ½ inch (1 cm) border all around. Gently and tightly roll the cake back up with the filling inside.
4. Transfer to a serving dish. Spread the remaining ganache all over the cake, using a rubber spatula or an offset spatula.

For the caramelized nuts:

1. Melt butter in a small sauté pan placed over medium heat. Add in the chopped nuts (hazelnuts and almonds), then toast them together with sugar and honey, about 1-2 minutes, stirring regularly. Add the orange peels in the end, sprinkle some cinnamon and stir well.
2. Top the yule log with the crispy caramelized nuts and cool in the refrigerator for about 1 hour. Remove 10 to 15 minutes before serving.