Preheat the oven to 350 degrees F.
Line the bottoms of three (9-inch) round baking pans with parchment paper.
Coat the parchment paper with baking spray.
In a medium bowl, add the flour, the baking soda, and 1 teaspoon of the salt and whisk to combine.
In the bowl of a stand mixer, add 1/2 cup of the softened butter, the vegetable shortening, and 2 cups of the sugar, creaming them together until light and fluffy.
Add the eggs, one at a time, beating well after each addition.
Add 2 teaspoons of the vanilla and mix to combine.
Add the dry ingredients mixture and the buttermilk to the creamed mixture, alternating between the two and adding about 1/3 of each at a time. Beat after each addition until smooth.
Fold in 1 1/2 cups of the chopped walnuts.
Evenly divide the batter among the three prepared baking pans.
Bake until a toothpick inserted near the center comes out clean, about 25-30 minutes.
Allow the cakes to cool in the pans for 8-10 minutes.
Transfer the cakes from the pans and allow them to cool completely on wire racks.
In a medium saucepan over medium heat, add 1/2 cup of the sugar and the water and cook, while stirring, until the sugar is dissolved, about 2-3 minutes.
Transfer the saucepan from the heat and stir in the cranberries until they are well-coated.
Using a colander, drain the cranberries.
Transfer the drained cranberries to a wire rack to dry, about 1 hour.
In small batches, roll the cranberries in the remaining 1 1/2 cups of the sugar until well-coated.
Let the cranberries dry, about 1 hour.
In the bowl of an electric mixer fitted with the paddle attachment, add the cream cheese, the remaining 1 1/2 cups of the softened butter, and 1/2 teaspoon of the kosher salt and beat them together, about 5 minutes, until light and smooth.
Beat the remaining 1 teaspoon of vanilla extract into the cream cheese mixture.
Add the confectioners' sugar into the cream cheese mixture, 1 cup at a time, mixing until smooth.
Add the eggnog to the cream cheese mixture, 1 tablespoon at a time, until you achieve the desired frosting consistency; you may not need to use all 3 tablespoons of the eggnog.
Level the cakes as needed with a serrated knife.
Spread 1 cup of the frosting between 2 layers of the cake.
Spread 1 cup of the frosting between the final two layers of the cake.
Spread the remaining frosting over the entire cake in a thin and even layer.
Serve the cake garnished with the sugared cranberries, the extra chopped walnuts, and the rosemary.