How to Make It
Prepare the Bourbon Custard Filling: Whisk together granulated sugar, cornstarch, and salt in a heavy saucepan. Whisk together half-and-half and egg yolks in a glass bowl. Gradually whisk half-and-half mixture into sugar mixture. Cook over medium, whisking constantly, until mixture starts to bubble, 7 to 8 minutes. Cook, whisking constantly, 1 minute; remove from heat. Whisk in butter and bourbon. Transfer to a medium bowl. Place plastic wrap directly on warm filling to prevent a film from forming. Let stand 30 minutes; chill 4 to 24 hours.
Prepare the Cake Layers: Preheat oven to 350°F. Beat butter with a heavy-duty electric stand mixer on medium speed until creamy. Gradually add granulated sugar, beating until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition. Stir together flour, baking powder, salt, cinnamon, and nutmeg in a large bowl. Stir together whole milk and eggnog in a glass measuring cup. Add flour mixture to butter mixture alternately with milk mixture, beginning and ending with flour mixture, beating on low speed just until blended after each addition. Stir in vanilla.
Divide batter evenly among 4 greased (with vegetable shortening) and floured 9-inch round cake pans. Bake in preheated oven until a wooden pick inserted in center of cakes comes out clean, 16 to 19 minutes. Cool in pans on wire racks 10 minutes. Transfer from pans to wire racks, and cool completely, about 1 hour.
Prepare the Eggnog Buttercream: Beat butter with a heavy-duty electric stand mixer on medium speed until creamy. Gradually add vanilla extract and 1 cup of the powdered sugar. Stir together eggnog and 1/2 cup of the cream in a small bowl. Gradually add remaining powdered sugar to butter mixture alternately with eggnog mixture, beating on low speed until blended after each addition. Beat on high speed until smooth and fluffy. Beat in up to 1/4 cup cream, 1 tablespoon at a time, if necessary to reach desired consistency.
Assemble the cake: Place 1 cup of the buttercream in a large piping bag. Place 1 Cake Layer on a serving platter; pipe a 1/2-inch-thick ring of buttercream on the very outer edge of the cake. Spread 1/3 cup of the filling inside the ring. Repeat procedure with 2 more layers. Top with fourth layer. Spread top and sides of cake with a very thin layer of buttercream. Chill 30 minutes.
Place remaining buttercream in a large piping bag fitted with a large star tip. Pipe 1 1/2-inch-wide tight zigzags up sides of cake, starting at the base each time and making sure each zigzag touches the one beside it. If desired, sprinkle cake with nutmeg.