Prepare the Sponge Cake

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a 10×15-inch jelly roll pan with parchment paper.
2. Beat Egg Yolks: In a large mixing bowl, beat egg yolks and sugar until thick and pale. Add vanilla extract.
3. Sift Dry Ingredients: In a separate bowl, sift together cocoa powder, flour, and salt. Gradually fold into the yolk mixture.
4. Whip Egg Whites: Beat egg whites in another bowl until stiff peaks form. Gently fold them into the batter in three parts to maintain the airy texture.
5. Bake: Pour the batter into the prepared pan, spreading it evenly. Bake for 12–15 minutes, or until the cake springs back when touched.

2. Roll the Cake

1. Dust with Sugar: Place a clean kitchen towel on a flat surface and dust it generously with powdered sugar.
2. Invert the Cake: Turn the warm cake out onto the sugared towel, peel off the parchment, and roll the cake tightly with the towel. Allow it to cool completely.

3. Make the Buttercream Frosting

1. Cream Butter: In a mixing bowl, beat the softened butter until creamy.
2. Add Dry Ingredients: Gradually add powdered sugar and cocoa powder, alternating with milk, until smooth and spreadable. Add vanilla for flavor.

4. Assemble the Buche de Noel

1. Unroll the Cake: Carefully unroll the cooled cake. Spread a layer of buttercream over the surface, leaving a small border.
2. Roll and Frost: Re-roll the cake without the towel. Frost the outside with the remaining buttercream, using a spatula to create a bark-like texture.
3. Decorate: Dust with powdered sugar for snow, add meringue mushrooms, and garnish with sugared cranberries or sprigs of rosemary.

5. Chill and Serve

Refrigerate the assembled cake for at least 30 minutes before serving to allow the flavors to meld and the frosting to set.