Whisk the flour, baking powder, nutmeg, and salt together in a large bowl.
In a separate bowl, whisk the brown butter, sugar, brown sugar, eggs, bourbon and, vanilla together until combined.
Stir in about half of the flour mixture. Then stir in the eggnog and the rest of the flour mixture and mix until fully combined. The batter will be thick.
Lightly spray three 6 inch layer cake pans with pan spray. Pour ⅓ of the batter into each cake pan (they will only be about half full). Bake at 350°F for 25-30 minutes or until a cake tester comes out clean.
Place a cooling rack on top of each cake pan and invert. The pan should slide cleanly away.
Let the cakes cool completely to room temperature before trimming.
Once the cakes are fully cooled. Using a sharp bread knife, trim the cakes until the layers are even. Wrap each layer tightly in plastic wrap and place in the freezer overnight to help the frosting spread on smoothly (minimum of 2 hours).
Brown Butter Frosting
In a large bowl using a handheld or stand mixer fitted with a paddle attachment, cream the room temperature brown butter and powdered sugar until smooth.
Add in the eggnog and bourbon and beat the frosting on high for 4-5 minutes until light and fluffy.
Milk Soak
In a small bowl, mix together the eggnog and bourbon and set aside.
Assemble the Cake
Remove cake layers from the freezer. Place the bottom layer of cake on your cake stand and brush with about ⅓ of the milk soak. Then spread about ¼ of the frosting on top in an even layer.
Then top with the next layer and repeat. Place the final cake layer upside down so that the top of your cake is as even as possible.
Smooth the rest of the frosting on the top and sides of your cake as desired.
Spoon the caramel sauce over the top and let it drip down the sides of the cake. If the caramel is too thick, microwave it for 5-10 seconds until the consistency of honey.
Refrigerate your cake for 2-3 hours before slicing and serving.
Notes
How to make brown butter: In a small saucepan, melt butter over medium heat. Once melted, stir frequently and watch closely for the formation of brown specs on the bottom of the pan and a nutty aroma. Once you see brown specs, stir for just a few more seconds (it browns super quickly). Remove from the heat and immediately pour into a different bowl to prevent it from burning. Let cool completely to room temperature. The brown butter should be solid but still pliable. You can leave the browned butter in an airtight container at room temperature overnight and it will be perfect for the next day! Note: For every ½ cup (1 stick or 113g), you'll lose about 1 tablespoon (14g) to evaporation while cooking. So you can either add in an extra 1 tablespoon before browning, or if you're browning big batches, just make up the weight difference in your recipe with regular unsalted butter if needed.
This recipe is designed for 3 six inch cake pans. It will also work with 2 eight inch cake pans.
Store the cake in the fridge for up to 4 days. However, it will taste best when served at room temperature! I like to let it sit at room temperature for about an hour before serving.