Prepare the Sponge Cake: Preheat the oven to 350°F (175°C). Line a jelly roll pan with parchment paper.

In a clean bowl, beat egg whites until soft peaks form, gradually adding 1/4 cup of the sugar until stiff peaks form.

In a separate bowl, beat egg yolks with the remaining 1/4 cup sugar until pale and thick. Stir in vanilla extract.

Sift together flour, cocoa powder, baking powder, and salt. Gently fold this mixture into the egg yolk mixture.

Carefully fold the beaten egg whites into the batter.

Pour batter into the prepared pan and spread evenly. Bake for 10-12 minutes.

Remove from oven, and while warm, roll the cake in a kitchen towel dusted with powdered sugar. Let it cool completely.

Prepare the Filling: In a chilled bowl, whip heavy cream, powdered sugar, and vanilla until stiff peaks form.

Roll and Fill the Cake: Unroll the cooled sponge cake and evenly spread the whipped cream filling.

Re-roll the cake tightly, wrap it in plastic wrap, and refrigerate for at least 1 hour.

Prepare the Chocolate Buttercream: Beat softened butter with powdered sugar, cocoa powder, and vanilla until fluffy. Add milk or cream for desired consistency.

Assemble the Yule Log: Unwrap the chilled cake and place it on a serving platter.

Frost with the chocolate buttercream, creating bark-like texture with a fork.

Decorate with cranberries, rosemary, and edible mushrooms. Dust with confectioners' sugar for a snowy effect.