PREPARE THE SPONGE CAKE: Preheat the oven to 350°F (175°C) and line a jelly roll pan with parchment paper. In a clean bowl, beat the egg whites until soft peaks form, gradually adding ¼ cup of the sugar until stiff peaks form. In a separate bowl, beat the egg yolks with the remaining ¼ cup of sugar until pale and thick, then stir in the vanilla extract. Sift the flour, cocoa powder, baking powder, and salt together and gently fold it into the egg yolk mixture. Carefully fold in the beaten egg whites without deflating the batter. Spread the batter evenly in the prepared pan and bake for 10–12 minutes, or until the cake springs back when touched. Immediately roll the warm cake in a clean kitchen towel dusted with powdered sugar, starting from the short end, and let it cool completely inside the towel.

PREPARE THE FILLING: In a chilled mixing bowl, whip the heavy cream with powdered sugar and vanilla until stiff peaks form. Once the cake is completely cooled, gently unroll it and spread the whipped cream evenly across the surface.

ROLL THE CAKE: Roll the cake tightly from the short end and wrap it in plastic wrap. Refrigerate for 1 hour to help it set before frosting.

MAKE THE CHOCOLATE BUTTERCREAM: In a bowl, beat the softened butter with powdered sugar, cocoa powder, milk or cream, and vanilla extract until smooth and fluffy. Use a spatula to spread the buttercream over the cake, then texture it with a fork to resemble tree bark.

ASSEMBLE THE YULE LOG: Unwrap the chilled rolled cake and place it on a serving platter. Frost it entirely with the chocolate buttercream, using a fork or spatula to create bark-like lines. Decorate with fresh cranberries, edible mushrooms, rosemary, and other seasonal items as desired.

FINAL TOUCH: Dust the entire cake with powdered sugar to resemble fresh snow before serving.