Preheat the oven to 350 degrees F. Grease and flour (or spray) two, 6 inch round cake pans that are 2 inches deep.
Set the butter out, cut it into chunks and allow it to come to room temperature. Don't microwave it, it needs to come to room temperature on its own.
In a large mixing bowl, add the dry ingredients (flour, sugar, baking powder and salt) and whisk together.
In another bowl, add the liquid ingredients (oil, eggs, sour cream, milk and vanilla) and whisk together until smooth.
Add the chunks of butter to the dry mixture slowly and mix with an electric mixer on medium speed as you add the butter. Mix until the mixture resembles a coarse, sand-like texture.
Add half the liquid to the dry mixture and mix on medium until well combined.
Add the second half of the liquid mixture and mix on medium until well combined.
Scrape down the sides of the bowl and then mix again just until all the ingredients are well incorporated. Be very careful not to over-mix. (Mixing for a minute or more is too long.) Batter will be a little thick.
Pour into prepared pans and smooth level. Bake at 350 degrees F for approximately 40-45 minutes. The cake layers are baked when they are golden brown and a toothpick inserted into the center of each layer comes out with a few moist crumbs on it, or clean, but not raw batter.
Cool on a wire rack, in the pans, for about 10-15 minutes, then turn the cakes out onto the racks and allow them to cool completely before covering with buttercream.
The tops of these cake layers will flatten out as they cool, but the tops can be leveled with a knife if needed in order to decorate.