1. Prepare the Chocolate Sheet Cake:

1. Preheat the oven to 350°F (175°C). Grease a 12×18-inch baking sheet and line it with parchment paper.
2. In a large bowl, whisk together the buttermilk, sour cream, eggs, vegetable oil, and vanilla extract until smooth.
3. Gradually mix in the chocolate cake mix until no lumps remain.
4. Pour the batter evenly into the prepared pan and smooth the surface.
5. Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean.
6. Let the cake cool completely in the pan.

2. Make the Whipped Cream Filling:

1. In a chilled mixing bowl, combine the heavy whipping cream, vanilla extract, and powdered sugar.
2. Beat with an electric mixer on high speed until stiff peaks form.
3. Refrigerate the whipped cream until needed.

3. Prepare the Chocolate Ganache Buttercream:

1. Heat 1 cup of heavy cream in a microwave-safe bowl until steaming but not boiling.
2. Pour the hot cream over the chocolate melts. Let it sit for 2 minutes, then stir until smooth and glossy.
3. Allow the ganache to cool slightly.
4. In a large bowl, beat the softened butter until creamy. Gradually add the cooled ganache, vanilla extract, and salt.
5. Slowly add powdered sugar, one cup at a time, and mix until smooth. If the buttercream feels too stiff, add a splash of heavy cream.

4. Assemble the Cake:

1. Carefully invert the cooled cake onto a large piece of parchment paper.
2. Spread the whipped cream filling evenly over the cake, leaving a 1-inch border around the edges.
3. Roll the cake tightly from one short end to the other, using the parchment paper to guide you. Place the rolled cake seam-side down on a tray.

5. Decorate the Yule Log:

1. Spread the chocolate ganache buttercream evenly over the rolled cake, creating a bark-like texture with a spatula or fork.
2. Dust the log with powdered sugar to resemble snow.
3. Decorate with sprigs of fresh rosemary and cranberries for a festive touch.