Preheat your oven to 335ºF and prepare three 6"x2" cake pans with cake goop or another preferred pan release. Make sure all of your ingredients are at room temperature.
Combine ½ of the buttermilk with the oil and set aside
To the remaining milk, add the eggs and the vanilla. Whisk to combine and set aside.
Combine the flour, sugar, baking powder, baking soda, salt, cinnamon, and nutmeg in the bowl of your stand mixer with the paddle attachment.
While mixing on low, add in your chunks of softend butter and donut fat and mix until it looks like coarse sand.
While mixing on low, add in the buttermilk/oil mixture and mix for 2 minutes on medium speed to develop the cakes structure.
While mixing on low, slowly drizzle in the milk/egg mixture until combined and then mix for another 30 seconds until the batter looks cohesive and smooth.
Divide the cake batter into your three pans and bake for 30-35 minutes or until a toothpick comes out clean from the center.
Immediately press the domes flat with a tea towel and then brush the surface of the cakes with the melted butter. Allow them to cool until barely warm in the pan then transfer to a cooling rack.
Flash freeze the cake layers for one hour before decorating