Prepare the Sponge Cake

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 15×10-inch jelly roll pan and line it with parchment paper.
2. Whisk Egg Yolks and Sugar: In a large bowl, whisk together egg yolks and 3/4 cup sugar until the mixture is light and fluffy. Stir in the vanilla extract.
3. Sift Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, salt, and cream of tartar.
4. Beat Egg Whites: In another bowl, beat the egg whites until soft peaks form. Gradually add the remaining 1/4 cup sugar and continue to beat until stiff peaks form.
5. Combine: Gently fold the dry ingredients into the egg yolk mixture. Then, fold in the egg whites, taking care to keep the batter light and airy.
6. Bake the Cake: Pour the batter into the prepared pan, spreading it evenly. Bake for 15-20 minutes, or until a toothpick inserted in the center comes out clean.
7. Cool: Once the cake is baked, remove it from the oven and let it cool for 5 minutes. Dust a clean kitchen towel with powdered sugar, then carefully turn the cake out onto the towel. Peel off the parchment paper and roll the cake up with the towel, starting from the short side. Allow it to cool completely in the rolled-up shape.

Make the Chocolate Filling

1. Heat the Cream: In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.
2. Melt the Chocolate: Place the chopped chocolate in a heatproof bowl. Pour the hot cream over the chocolate and stir until smooth. Stir in the vanilla extract. Let the mixture cool to room temperature.

Make the Ganache

1. Heat the Cream: In another saucepan, heat the remaining heavy cream until it begins to simmer.
2. Melt the Chocolate: Place the chopped chocolate in a heatproof bowl and pour the hot cream over it. Stir until the chocolate is completely melted and smooth. Stir in the butter until incorporated. Let the ganache cool slightly before using.

Assemble the Yule Log

1. Unroll the Cake: Once the sponge cake has cooled completely, carefully unroll it from the towel.
2. Fill with Chocolate: Spread the chocolate filling evenly over the surface of the cake, leaving a small border around the edges.
3. Re-roll the Cake: Carefully roll the cake back up, this time without the towel. Place the cake seam-side down on a serving platter.
4. Frost the Cake: Spread the chocolate ganache over the entire log, smoothing it with a spatula. If desired, use a fork to create bark-like texture on the surface of the ganache.
5. Decorate: If you’re feeling festive, dust the Yule Log with powdered sugar to give it a snowy effect. Add holly leaves, mint sprigs, or even mini meringue mushrooms to decorate the cake and make it look like a winter wonderland.