To make the cake layers:

1. Using a mixer with a beater/paddle attachment (not a whisk), beat room temperature butter and sugar on medium speed until pale and creamy, about 3 minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter or sugar is stuck at the bottom.
2. Add room temperature eggs one at a time, mixing on the lowest speed after each addition for 30 seconds. Scrape down to the bottom of the bowl to make sure everything is incorporated before adding the next egg.
3. Add vanilla and mix for about 30 seconds to combine. Add room temperature buttermilk and oil and mix on the lowest speed to incorporate.
4. In a bowl, sift flour, baking soda, baking powder and salt. Add to the mixing bowl and mix on lowest speed to incorporate, scraping down to the bottom of the mixing bowl to check that batter is evenly mixed.
5. Grease three 8" baking pans with non-stick oil spray or butter. Divide batter equally between the cake pans.
6. Bake at 160ºC or 325ºF for 35 minutes or until the cake bounces back up when you poke it.
7. Leave cakes to cool in their pans for 10 minutes. Use a spatula to loosen the cakes from the edges of the pans and then turn onto a wire cooling rack. Leave to cool completely.
8. Optionally, when the cakes have cooled you can use a serrated knife like a bread knife to trim off any domed tops of the cake layers to leave them level.

To make the eggnog frosting:

1. Put flour, milk and salt in a pan and cook over medium heat for about 5 minutes. Whisk it constantly until it simmers to prevent lumps. When the mixture thickens, switch to a spatula. Scrape along the bottom and sides of the pan, stirring for about two minutes until the mixture thickens enough to leave ribbons when you lift the spatula.
2. Remove the pan from the heat and stir in vanilla, nutmeg and cinnamon. Pour the mixture into a bowl and cover it with plastic wrap (e.g. cling film / Saran Wrap), pressing it down to touch the surface of the mixture. Put it into the fridge for about an hour to cool to room temperature. 
3. Mix butter and sugar at medium speed until creamy and lighter in colour, about 3 minutes. Add cooled flour mixture and mix for another minute, until smooth.

To make the spiked simple syrup:

1. Mix water, sugar and rum, brandy or bourbon in a pan. Bring it to a simmer over medium heat, stirring occasionally until the sugar dissolves. Let this cool and then drizzle generously over cake layers with a squeezy bottle or brush with a pastry brush.

Then pour it into a squeezy bottle or use a pastry brush to drizzle or brush this over your cake layers. You can do this on the top of the layers as they are, or if they’re domed, level the tops with a serrated knife and the syrup will absorb much better like this.

To put the eggnog cake together:

1. Spread a bit of eggnog frosting onto the middle of a cake board or plate and press your first cake layer down to attach it. Spread eggnog frosting to cover the top of the cake and then place another cake layer on top. Repeat with another layer of eggnog frosting and the final cake layer.
2. Put the cake in the fridge for 30 minutes or the freezer for 15 minutes to chill and set. Then spoon half of the remaining eggnog frosting on top and spread with an offset spatula or palette knife to cover the cake. Use a straight edged cake comb or frosting smoother to smooth the frosting but it doesn't need to be perfect yet! Chill in the fridge for another 30 minutes or freezer for 15 minutes to set this crumb coat of frosting.
3. Spoon the rest of the eggnog frosting onto the cake and spread to cover the cake. Smooth with a cake comb and optionally, add piping around the top using a star shaped tip like a #32.
4. You can refrigerate this cake for 2-3 days but take it out 2 hours before serving so that the cake comes to room temperature.