Prepare the Cake:

• Preheat the oven to 350°F (175°C). Grease and line a 10×15-inch jelly roll pan with parchment paper.

• In a medium bowl, sift together the flour, cocoa powder, baking powder, and salt.

• In a large bowl, beat the egg yolks and sugar until thick and pale. Add the vanilla extract and water, mixing until combined.

• Fold in the dry ingredients until just combined.

• In a separate bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the batter until no white streaks remain.

• Spread the batter evenly into the prepared pan.

• Bake for 10–12 minutes, or until the cake springs back when lightly touched.

• While the cake bakes, prepare a clean kitchen towel by dusting it with powdered sugar.

• Once baked, immediately invert the cake onto the prepared towel. Carefully peel off the parchment paper. Starting from one short end, gently roll the cake with the towel inside. Allow it to cool completely.

Make the Chocolate Buttercream:

• Melt the semisweet chocolate in a heatproof bowl over simmering water or in the microwave. Let it cool slightly.

• In a large bowl, beat the softened butter until creamy.

• Gradually add the powdered sugar and cocoa powder, beating until smooth.

• Add the melted chocolate and vanilla extract, mixing until well combined and fluffy.

Prepare the Ganache:

• In a small saucepan, heat the heavy cream until it just begins to simmer.

• Pour the hot cream over the chopped semisweet chocolate in a heatproof bowl. Let it sit for 2 minutes, then stir until smooth and glossy.

• Add the butter to the ganache, stirring until melted and fully incorporated. Let it cool to room temperature.

Assemble the Yule Log:

• Carefully unroll the cooled cake. Spread an even layer of the chocolate buttercream over the entire surface.

• Re-roll the cake without the towel, starting from one short end.

• Place the rolled cake seam-side down on a serving platter.

• Spread the ganache evenly over the top and sides of the cake. Use a fork or spatula to create a bark-like texture.

Decorate:

• Dust the top of the cake with powdered sugar to resemble snow.

• Optional: decorate with meringue mushrooms, fresh berries, or rosemary sprigs for a festive touch.

Serve:

• Chill the Yule Log in the refrigerator for at least 1 hour before serving to allow the ganache to set.

• Slice into generous rounds and serve on a festive platter.