Begin by preheating your oven to 350 degrees F. Line the bottoms of three 9-inch round baking pans with parchment paper. Spray with baking spray. Set aside.
In a medium bowl, whisk together the flour, baking soda, and salt to combine. Set aside.
In the bowl of a stand mixer, cream together the butter, vegetable shortening, and sugar. Mix until light and fluffy. Add the eggs, one at a time, beating after each addition. Add in the vanilla. Mix to combine.
Add the dry ingredients to the creamed mixture, alternating with the buttermilk. Beat after each addition until smooth. Fold in the walnuts.
Evenly divide the batter among the three prepared baking bans. Bake for 25-30 minutes or until a toothpick interested near the center comes out clean. Cool the cake in the pans for 8-10 minutes; then remove to wire racks to cool completely.
In the meantime, prepare cream cheese frosting* using the recipe linked below. Eliminate the almond extract and substitute the milk with eggnog for a festive touch.
Once the cakes have cooled, level each with a serrated knife, if necessary. Then spread about 1 cup of frosting between the layers of cake. Add another cup of frosting to the top of the cake. Spread over the cake in an even, thin layer,** using an offset spatula.
Top with additional black walnut pieces, sugared cranberries***, and sprigs of fresh rosemary to garnish.