Eggnog Spice Cake with Bourbon Custard Filling and Eggnog Buttercream

Name: Eggnog Spice Cake with Bourbon Custard Filling and Eggnog Buttercream
Cook time: 6 h
Total time: 6 h
Cuisine: American
Ingredients: .5 cup granulated sugar; .25 cup cornstarch; .25 teaspoon salt; 2 cups half-and-half; 4 large egg yolks; 3 tablespoons salted butter; 2 tablespoons bourbon; 1 cup salted butter, softened; 2 cups granulated sugar; 4 large eggs; 3.25 cups all-purpose flour, plus more for dusting pans; 2.5 teaspoon baking powder; .75 teaspoon salt; .5 teaspoon ground cinnamon; .25 teaspoon ground nutmeg; .75 cup whole milk; .5 cup refrigerated eggnog; 1 teaspoon vanilla extract; Vegetable shortening, for greasing pans; 1 (2-lb.) package powdered sugar, divided; .25 cup refrigerated eggnog; .5 - .75 cup heavy cream, divided; Grated fresh nutmeg (optional)