Preheat oven to 350F and grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, nutmeg, cinnamon, and salt. Set aside.
Using a stand mixer fitted with the paddle attachment, beat butter until smooth. Add sugars and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and eggnog, beginning and ending with flour (3 additions of flour and 2 of eggnog). Fully incorporating after each addition. Do not overmix.
Spread batter evenly into prepared pans. Smooth the tops with a spatula.
Bake for approx. 30-35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack. Allow cakes to cool completely.
Eggnog Latte Soak:
Combine eggnog and espresso powder until dissolved. Heat if necessary to help dissolve but cool completely before use.
Espresso & Eggnog Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.***
Divide buttercream in half. Add eggnog and nutmeg to one half and whip until smooth. Add espresso powder to the other half and whip until smooth.****
Trim the tops of each cake layer to remove crust. Brush each with the eggnog latte soak.
Place one layer of cake on a cake stand or serving plate. Top with about 2/3 cup of espresso buttercream. Repeat with remaining layers and apply a thin coat of frosting all over the cake. Chill for 20mins.
Starting with the bottom and the espresso buttercream, spread roughly onto bottom half of cake with a small offset spatula. Spread eggnog buttercream on top of the cake and the upper half. Use a large icing smoother to smooth the sides and blend the buttercreams. Smooth the top with a small offset spatula.
Fit a piping bag with a Wilton Tip 1M piping tip. Spread the Espresso buttercream on one side of the piping bag and the eggnog buttercream on the other side. Pipe dollops on top of the cake.
Sprinkle with nutmeg top with Nutmeg and small white nonpareils and line the bottom with white chocolate and salted caramel crispearls if desired (for color).
* For the cake recipe, if your eggnog is on the thicker side (thicker than cream), use 1 cup eggnog and 1/4 cup milk to think it out.
** Wipe your mixer bowl and whisk down with lemon juice or vinegar to make sure it is completely grease free and make sure there is no trace of yolk in your whites or your meringue will not stiffen.
*** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. If it looks soupy, place it in the fridge for 20mins and rewhip.
***** If your instant espresso is not a superfine powder, I recommend dissolving it first in 1-2 tsp hot water and cooling before adding to the frosting.