Preheat your oven to 325°F (163°C) and grease a 9x13 inch baking dish.

In a large bowl, whisk together the buttermilk, sour cream, eggs, vegetable oil, and vanilla extract until smooth. Gradually sift in the chocolate cake mix, mixing gently until just combined.

Pour the batter into the prepared baking dish and bake for 27-30 minutes. Ensure a toothpick comes out clean when inserted in the center. Let cool completely on a wire rack.

For the whipped cream filling, whip the heavy cream in a chilled bowl until stiff peaks form (3-5 minutes). Add vanilla extract and powdered sugar, mixing until combined.

Slice the cooled cake in half horizontally. Spread whipped cream filling on one half and place the top half back on. Chill for 30 minutes.

For the ganache buttercream, heat chocolate melts and heavy cream in the microwave, stirring until smooth. Beat in unsalted butter and salt, then add powdered sugar until fluffy.

Spread the ganache buttercream on the top and sides of the cake, creating a log-like texture. Decorate with powdered sugar, rosemary, and cranberries before serving.