Preheat oven to 325F.
Give three eight-inch round cake pans a coating of butter or vegetable oil.
Then, cover the bottoms with rounds of parchment paper, and lightly oil the parchment paper as well.
Set those pans aside for now.
In a medium-sized mixing bowl, whisk together the all-purpose flour, vanilla instant pudding mix, baking soda, cocoa powder, baking powder, and Kosher salt until evenly distributed.
In a large mixing bowl, use a hand mixer or stand mixer to beat the whole eggs, white granulated sugar, whole milk, buttermilk, vegetable oil, melted butter, and vanilla extract until frothy and relatively homogenous.
Add the dry ingredients to the wet ingredients, and use a hand mixer to blend the ingredients until they are smooth.
Evenly distribute the cake batter across the three cake pans that you prepared earlier.
Bake at 325F for roughly 30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
When the cakes are fully baked, remove them from the oven, and set them aside at room temperature to cool down.
When the pans have cooled for about 20 minutes, carefully invert them onto wire racks to allow them to completely cool down for an additional 60 minutes.