Eggnog Layer Cake with Bourbon-Vanilla Bean Buttercream Frosting

Prepare the Cake Layers: Preheat oven to 350°F. Coat 4 (6-inch) round cake pans with baking spray. Line each pan bottom with a circle of parchment paper; coat parchment with baking spray. 

Beat sugar and butter with a stand mixer fitted with a paddle attachment on medium speed until mixture is light and fluffy, about 4 minutes. Add eggs 1 at a time, beating until combined.

Stir together flour, baking powder, salt, and nutmeg in a separate bowl. Add flour mixture to sugar-butter mixture alternately with 1 cup of the eggnog in 3 additions, beginning and ending with flour mixture. Beat on low speed until blended after each addition, about 1 minute total. Beat in bourbon and vanilla extract until smooth. Divide batter evenly among prepared pans (1⅔ cups per pan). 

Bake on center rack in preheated oven until a wooden pick inserted in center of each layer comes out clean, 25 to 28 minutes. Cool in pans 15 minutes. Run an offset spatula around edges of each layer; remove from pans, and cool completely on a wire rack, about 45 minutes. Using a long serrated knife, trim rounded tops from each layer (they should be about 1¼ inches tall after trimming). Drizzle evenly with remaining 1 cup eggnog (¼ cup per layer). Let stand 15 minutes.

Meanwhile, prepare the Frosting: Beat butter with an electric mixer on medium speed until creamy, about 2 minutes. Reduce mixer speed to low. Gradually beat in powdered sugar alternately with cream, beating until blended after each addition, about 2 minutes total. Scrape seeds from split vanilla bean pod using dull edge of a paring knife; add scraped vanilla seeds and bourbon to butter mixture (discard scraped vanilla bean pod). Beat on high speed until smooth and fluffy, about 2 minutes.

Place 1 Cake Layer, cut side up, on a platter; spread ⅔ cup Frosting over top. Top with a second layer, cut side up, and another ⅔ cup Frosting. Top with a third layer, cut side up, ⅔ cup Frosting, and then remaining layer, cut side down. Frost top and sides of cake with 2 ½ cups Frosting, reserving remaining ½ cup Frosting for tree decoration.

With a small paring knife or a wooden pick, draw an outline of a Christmas tree on side of assembled cake. Using an offset spatula, scoop teaspoonfuls of reserved ½ cup Frosting, and place on cake, 1 at a time, starting at the bottom of the tree outline. With a spatula, use a swooping motion to create a brushstroke-like branch with each dollop of Frosting, moving from the inside of the tree to the outside. Continue scooping and swooping with spatula, working from bottom to top of tree and from inside to outside, overlapping as needed, until tree is complete. Decorate tree as desired with assorted candy sprinkles, nonpareils, and sparkling sugar.

Place 1 Star Shortbread Cookie at the top of the tree, and place several Present Shortbread Cookies around the base of the cake.